NEAR-EARTH OBJECT EXTRAVEHICULAR ACTIVITIES:
USING APOLLO AND ISS OPERATIONS TO MAP LOW-GRAVITY TERRESTRIAL
SPACEWALK OBJECTIVES AND CHALLENGES
Matthew A. Gast
Georgia Institute of Technology
School of Aerospace Engineering
Atlanta, GA 30332-0150
[email protected]
David A. Spencer
Georgia Institute of Technology
School of Aerospace Engineering
Atlanta, GA 30332-0150
[email protected]
Abstract
The notion of human exploration of a near-Earth object (NEO) is nothing new. Jules Verne wrote about this very
idea in his story “Off on a Comet,” first published in France in 1877. Since that time, a number of studies have
examined NEO exploration for scientific purposes, in-situ resource utilization, mineralogical exploitation and even
planetary defense; as early as 1966, a study was conducted to utilize the Apollo program hardware to fly by asteroid
Eros 433 [32]. Yet there is very little in the literature archive addressing extra-vehicular activities operations on the
surface of a near-Earth object. The arguments for manned missions to near-Earth objects have been presented in a
number of papers, recognizing astronauts’ adaptability to real-time challenges, the capability to collect geological
samples while identifying the overall geological context, and the ability to return a great quantity of those geological
samples to Earth, as just a few of the many reasons for a NEO manned mission. Few studies, however, have
identified or discussed the myriad challenges of performing surface operations in an environment where the
gravitation is considerably less than that of the Moon, but not negligible like the micro-gravity of an International
Space Station (ISS) – based EVA. Using the operational experience learned from NASA’s various human
exploration programs, this paper will identify key challenges unique to NEO surface operations. Furthermore, this
paper will map the applicable EVA tasks from both the Apollo program’s lunar exploration missions and ISS
construction to present an EVA operational concept for NEO surface exploration. Through mapping the applicable
Apollo and ISS tasks to the surface of a NEO, relevant operational objectives and challenges are identified, and
conceptual approaches to meeting the NEO EVA mission objectives and mitigating key risks are discussed.
1.
Introduction
One of the greatest challenges with any manned
spaceflight program is defining realistic operational
concepts – and thus the hardware to support these
operations – early in the program’s development.
Often the funding and timetable dictate that some sort
of basic concept be developed, knowing full well it
will need to be iterated throughout the program’s
lifecycle. This paper will attempt to do a small
number of things with respect to extra-vehicular
activities (EVA; i.e. spacewalking) operations
performed on a near-Earth object (NEO). This paper
provides a conceptual roadmap for the development
of a NEO EVA architecture drawing upon historical
EVA objectives and best practices. First, operational
best practices from space shuttle and International
Space Station (ISS) EVAs are identified, as well as
applicable recommendations made by the Exploration
Systems Architecture Study (ESAS) team for the
Constellation program [33]. By referencing current
operational constraints and best practices, it is
possible to establish a meaningful starting point for
defining a NEO EVA exploration architecture. Next,
the paper will attempt to identify the operational
concepts of a NEO EVA by mapping the applicable
surface activities performed on the Moon by the
Apollo astronauts to a near-Earth object mission. By
categorizing the various activities performed on the
Moon, this paper will then present the challenges that
each category of activity will face when performed
on a near-Earth object.
Finally, this paper will discuss mitigation steps
for each category of operational risk. One crosscutting risk that applies to multiple NEO EVA
operations, astronaut mobility, will be discussed in
detail. By focusing in greater detail upon surface
mobility, this paper will then present a number of
issues that will require resolution. Furthermore, this
paper will demonstrate one simplistic way to address
these issues using the ISS as an EVA research
facility. Operating in the microgravity vacuum of
low-Earth orbit, the ISS provides an exceptional testbed to validate designs and concepts. This paper will
try to present some of the tests that may lead to more
effective exploration beyond the Earth-Moon system.
In the 45 years since Alexei Leonov’s first
extravehicular activity (EVA) of 12 minutes and nine
seconds aboard Voskhod 2 on March 18, 1965,
people from a multitude of nations have logged
hundreds of hours of spacewalk time. Human
spacewalking experience, however, can be classified
into just two distinct environments: the micro-gravity
of space in low-Earth orbit, and the one-sixth-gravity
of the lunar surface. [Note: for simplicity this paper
will refer to all EVAs conducted in the micro-gravity
environment of space as orbital EVAs, meaning not
conducted on the surface of a celestial body.]
Currently, the vast majority of spacewalking
experience, knowledge and skill has come from
orbital EVAs, but this was not always the case.
When Apollo 11 launched to the Moon as the
first designated mission to land on its surface, NASA
had just 13 hours and 40 minutes of orbital EVA
experience, and not a single minute of experience on
an extra-terrestrial body. Neil Armstrong and Edwin
(Buzz) Aldrin would be forced to rely on their Earthbased training, which was designed to merge the
knowledge gained through the orbital EVAs of the
Gemini program and Apollo 9 with the low-gravity
operational concepts developed specifically for
Apollo’s lunar surface program. By the time Gene
Cernan stepped off the Moon and onto the ladder of
the Lunar Module (LM) for the last time on Apollo
17, NASA astronauts had accumulated approximately
161 man-hours of lunar spacewalking experience.
In 1972, NASA had an order of magnitude more
hours of EVA experience on the lunar surface than in
the micro-gravity environment of space. With the
cancellation of the Apollo program, and the
subsequent launch of Skylab in 1973, however, it
became apparent that expanding EVA capabilities in
orbit was essential. The success of Skylab itself
hinged on the crews’ ability to perform an EVA to
repair a solar array that had been damaged during
ascent [27]. This event brought the adaptability and
utility of EVA to the forefront of mission design.
EVA capabilities for the Space Shuttle program
were initially considered only necessary for
contingency tasks, yet again its usefulness and
flexibility proved invaluable. When the operational
concepts for Space Station Freedom were outlined,
the space shuttle’s payload bay became a proving
ground for engineering-based EVA activities. On
space shuttle missions like STS-37, flown in April of
1991, astronauts performed planned EVAs to test
hardware meant for use on a future space station,
allowing the engineers on the ground to gain flight
experience [8]. From the knowledge gained through
astronaut feedback, designs were altered, concepts
were deemed acceptable, and the foundation for the
construction of the largest, most complex space
system was laid.
This shifted the majority of experience away
from lunar EVAs and back to orbital EVAs. The
successes of the Hubble Space Telescope servicing
missions gave further credibility to EVA,
demonstrating the exacting abilities of astronauts
garbed in pressurized space suits to perform
sensitive, detail-oriented work.
The challenge of the International Space Station
(ISS) construction, however, ushered in the golden
age of spacewalking. The assembly of ISS alone has
required 134 separate EVAs thus far (as of STS131/19A completion), with U.S. astronauts
dedicating approximately 1,787 man-hours to ISS
construction. Over the course of four years, from
1969 to 1972, six separate crews visited the surface
of the Moon, conducting a total of 14 EVAs. To
contrast this, NASA conducted as many as 23
separate, distinct EVAs in 2002 alone, and matched
this number again in 2007 [8].
Yet even after 45 years of spacewalking, the
experience is limited to these two environments.
There is no doubt, however, that the destinations of
the future are beyond low-Earth orbit, and some EVA
environments will be beyond the current
spacewalking experience-base. Martian gravity, for
example, is approximately one-third that of Earth
(twice that of the Moon), so it is logical to imagine
the operational concept for Mars EVA will be some
interpolation of techniques used by the Apollo
astronauts on the surface of the Moon and techniques
developed here on Earth as part of future mission
preparation field tests. Human exploration of Mars
however, is likely several decades in the future. In
the nearer-term, within the next two decades, nearEarth asteroids represent the next logical step in
human exploration beyond the Earth-Moon system.
The question, then, revolves around the types of
challenges that will be associated with feasible
destinations.
Traveling to any potential target
beyond the Earth-Moon system (i.e., a near-Earth
object, the moons of Mars, the asteroid belt) will take
considerable time. It is essential, therefore, that a
manned mission to any such target possess a wide
variety of capabilities, to ensure that any operational
challenges presented by the uniqueness of the target –
from composition, to rotation rate, to varying
gravitational fields – do not force the astronauts to
abort a mission without exploring the target’s
surface.
2.
An Exploration Strategy
For many, Mars represents the logical next step
in human space exploration. The myriad challenges
of Martian exploration provide an opportunity for
great innovation, but charging forward with Mars as
the ultimate goal sets up a repeat of the Apollo
program; public interest and government funding
tend to wane upon the successful achievement of a
seemingly insurmountable goal.
Instead, the exploration strategy must be founded
upon a progressive, evolutionary approach, where
each new experience adds capabilities to the
exploration toolbox. In this way, core competencies
do not become destination specific, but are
transferable and act only to enhance the exploration
architecture.
While human spaceflight currently possesses a
unique set of EVA skills, very low-gravity terrestrial
exploration will require something more.
The
construction of the ISS in microgravity was possible
only because of the operational considerations
included in the design. Translation about the ISS
would not be possible without countless handrails
along every potential translation path, and assembly
would not have been possible without body restraints
such as foot plates and the Space Station Remote
Manipulator System (SSRMS). Without restraint
systems, astronauts would have had no way to react
the loads induced, for example, from bolting together
truss segments.
The exploration of a very low-gravity body, in
contrast, presents nearly all the challenges of
microgravity EVA, but without the man-made
luxuries that made ISS construction possible. In
addition, the EVA tasks of the Apollo program that
were relatively simple to perform on the Moon –
taking core samples, retrieving surface samples, and
even walking – become complicated on the surface of
a very low-gravity body. Thus, the current EVA core
competencies provide an excellent starting point for
the exploration of very low-gravity bodies, but it is
apparent that both operational concepts and hardware
will need to be developed to explore the vast array of
very low-gravity bodies scattered throughout the
solar system.
To assist in the development of said operational
concepts and hardware, a NEO exploration
architecture should utilize robotic precursor missions
in conjunction with manned missions, to gain
experience incrementally in such areas as target
reconnaissance, communication latency, crew
autonomy and resource management. To ensure each
manned mission is fully prepared, the robotic oneway precursor missions could be sent to any number
of NEOs of interest, to determine the characteristics
of each NEO – such as its gravitational field, general
composition and rotation rate – and to assess whether
a specific NEO is a feasible candidate for human
exploration [2,17,24].
Incorporating robotic
precursor missions into the exploration architecture
would mean that a single robotic vehicle design could
be the template for a series of missions, utilizing
economies-of-scale in searching for NEO candidates
by building a number of robotic vehicles from one
design, thus reducing the cost per launch.
The exploration of Mars remains a monumental
goal, but considering the nearly limitless number of
other potential targets for exploration, Mars cannot
be the end point of an exploration strategy. Instead,
the strategy must develop the EVA core
competencies that will allow mankind to explore any
terrestrial body. By developing the capabilities to
explore such destinations as near-Earth objects and
the moons of Mars, destinations beyond Mars – such
as main-belt asteroids, and the moons of Jupiter and
Saturn – become the potential targets of the future.
In this way, Mars becomes a part of the exploration
strategy, rather than the final destination of a unique
exploration program. And all of this is possible by
using NEO exploration as the backbone of an
evolutionary exploration architecture that makes each
and every destination in the solar system a steppingstone for the next.
3.
Near-Earth Object Selection
The question now being asked is “what
constitutes the ideal near-Earth object for a manned
mission?” The response, of course, is that it depends.
With the wide variety of near-Earth object types, and
the vast majority of the total NEO population yet to
even be discovered, much less characterized, it
depends on which factors have the greatest influence
upon the mission profile. Is scientific exploration the
driving factor, or is it bounding the mission duration
within a timeframe that reflects current operational
expertise? Is it more important to maximize the
duration of proximity operations, or to reach the NEO
using the smallest possible Δv? Until these questions
are definitively answered, any attempt to develop a
design reference mission will require it be based
upon a large number of assumptions.
To begin to understand the challenges associated
with NEO exploration, it is necessary to identify the
array of bodies that make up the NEO population,
and note the ways in which they are different from
one another. This paper cannot begin to capture the
diversity of the NEO population as it is currently
understood, but will present a sampling of the
variations on key characteristics of NEOs to give an
appreciation for the challenge of defining and
developing a single exploration architecture that can
investigate each type of body with adequate
capability.
The long-standing definition of a near-Earth
objects had been any solar system body (i.e. asteroid
or comet) whose orbital elements met the following
qualifications:
perihelion
distance
of
1.3
Astronomical Units (AU) or less, and aphelion
distance of 0.983 or more [28]. In February of 2003,
the Lincoln Near-Earth Asteroid Research (LINEAR)
program at MIT’s Lincoln Laboratory discovered the
first of a new class of near-Earth objects, that of an
inner-Earth asteroid (IEA), whose entire orbit is
within the Earth’s orbit [10]. Since then, according
the NASA JPL Small-Body Database (as of May,
2010), nine more IEAs have been discovered, the last
of which, asteroid 2008 UL90, was discovered in
October of 2008 [38]. Table 1 shows the delineation
of the NEO groups, based on the such parameters as
perihelion and aphelion distances, orbital period (in
the case of near-Earth comets) and semi-major axis.
As indicated in Table 1, asteroids make up the
majority of the NEO population. As such, much of
this paper will often focus on asteroids, in essence
using the terms near-Earth object and near-Earth
asteroid interchangeably.
Near-Earth comets,
however, remain viable and desirable targets, but it is
likely that any general system that can address the
various classes of near-Earth asteroids will have
applicability to comet exploration.
Near-Earth asteroid taxonomy places asteroids
(and the meteorites that fall to Earth) into three main
categories. This categorization is generally accepted
throughout the scientific community, but beyond the
three main groups, much debate continues regarding
how to further delineate the variety of asteroid types
that have been discovered thus far. Table 2 below
shows the three primary categories, and the types of
asteroids found in each category, as defined by Bus
and Binzel [5].
Category
S-Complex
C-Complex
Group
Description
Definition
NEC
Near-Earth Comet
NEA
Atira
Near-Earth Asteroid
NEAs whose orbits are
contained entirely within the
orbit of the Earth (named after
asteroid 163693 Atira).
Earth-crossing NEAs with
semi-major axes smaller than
Earth’s (named after asteroid
2062 Atens).
Earth-crossing NEAs with
semi-major axes larger than
Earth’s (named after asteroid
1862 Apollo).
Earth-approaching NEAs with
orbits exterior to Earth’s but
interior to Mars’ (named after
asteroid 1221 Amor).
Potentially
Hazardous
Asteroids:
NEAs
whose
Minimum Orbit Intersection
Distance (MOID) with the
Earth is 0.05 AU or less and
whose absolute magnitude (H)
is 22.0 or brighter.
q<1.3 AU,
P<200 years
q<1.3 AU
Q<0.983 AU
Aten
Apollo
Amor
PHAs
a<1.0 AU
Q>0.983 AU
a>1.0 AU
q<1.017 AU
a>1.0 AU
1.017 < q <
1.3 AU
MOID
<=
0.05 AU
H<=22.0
Table 1. NEO Groups – The vast majority of NEOs are asteroids,
referred to as Near-Earth Asteroids (NEA). NEAs are divided into
groups according to the perihelion distance (q), aphelion distance
(Q) and semi-major axis (a) , and in the case of Near-Earth Comets
(NECs), the orbital period (P) [37]
X-Complex
Class
A, Q, R, K, L,
S, Sa, Sk, Sl,
Sq, Sr
B, C, Cb, Cg,
Cgh, Ch,
X, Xc, Xe, Xk
General Trait
Silicaceous
(stony)
Carbonaceous
Metallic (most
often Ni-Fe)
Table 2: Asteroid Taxonomy [5]
C-complex asteroids are believed to be the
largest population by percentage, accounting for over
one-half of all asteroids. Next would be the Scomplex asteroids, and finally the metallic Xcomplex asteroids, which account for just 7% of the
asteroids discovered thus far [5].
Discovering near-Earth objects has become a
national endeavor. In 2005, Congress enacted the
NASA Authorization Act [1], which stated:
The Administrator [of NASA] shall plan,
develop, and implement a Near-Earth Object Survey
program to detect, track, catalogue, and characterize
the physical characteristics of near-Earth objects
equal to or greater than 140 meters in diameter in
order to assess the threat of such near-Earth objects
to the Earth. It shall be the goal of the Survey
program to achieve 90 percent completion of its
near-Earth object catalogue (based on statistically
predicted populations of near-Earth objects) within
15 years after the date of the enactment of this Act.
Thus by 2020, NASA is to have identified and
characterized 90 percent of the NEO population, a
population which is believed to approach 100,000
individual objects, of which approximately 20,000
could be considered Potentially Hazardous Asteroids
(PHAs) [34].
The methods employed are working; the number
of known NEOs is constantly growing. Abell et al.
noted that, as of September 29, 2009, 6,482 NEOs
had been catalogued, including 1,072 PHAs [2]. Just
seven months later, and as of May 06, 2010, NASA
reports cataloguing 6,918 NEOs, including 1,118
PHAs [37].
As more surveying tools come online, like the
Large Synoptic Survey Telescope, being built for
deployment to Cerro Pachon in northern Chile, and
capable of detecting NEOs within the main asteroid
belt as small as 140 meters in less than one minute
[39], the known population of near-Earth objects will
grow even more rapidly. The LSST is slated to begin
construction in 2011 with early scientific operations
starting in 2016 [22], and could prove to be a
valuable tool to characterize target NEOs, to identify
a subset worthy of robotic precursor exploration.
Figure 1 shows the total known NEA population
through June 2010. NASA’s Near Earth Object
Program has catalogued over 7,000 objects, with
approximately 800 having a diameter larger than one
kilometer. Most significantly, notice the rapid rate of
discovery of asteroids with estimated diameters of
less than one kilometer, and compare that discovery
rate to that of the large NEA category, with estimated
diameters of one kilometer or larger. Jedicke et al.
suggested that using current surveying methods, 80%
of the estimated 1000 large NEAs would be
discovered by 2008; this chart supports that
hypothesis [13].
Figure 2 shows the distribution of NEA sizes that
have been discovered thus far.
The largest
population of asteroids falls into the 300-meter to
1,000-meter diameter category. However, there are a
significant number of large NEAs as well.
Finally, one additional note on target selection.
It has been suggested that to avoid the need for
significant
launch
vehicle
and
spacecraft
performance, any NEO that could be considered for
exploration should have a very small ecliptic
inclination, on the order of 5° or less [3]. As the
population of known NEOs grows, this suggested
constraint may or may not have a significant impact
on target selection and/or vehicle design.
Figure 1. NEA population (“large” is defined as having a diameter
greater than one kilometer)[37]
Figure 2. NEA Distribution by Diameter [37]
4.
Spacecraft EVA Capabilities
The Apollo program utilized a crew of three to
explore the Moon; once in low-lunar orbit, the
Commander (CDR) and the Lunar Module Pilot
(LMP) would transfer to the Lunar Module (LM),
while the Command Module Pilot (CMP) remained
behind to man the orbital spacecraft that would bring
the crew back to Earth. The LM would separate from
the Command Module, and the CDR and LMP would
descend to the surface of the Moon.
Once on the lunar surface, the two-person crew
would don the Apollo space suits and, over a number
of hours, prepare for surface operations. When all
systems were GO for EVA, the crew would depress
the entire pressurized volume of the LM, and once at
vacuum, open the hatch, egress onto the porch and
descend down the ladder to the lunar surface.
The utilization of an airlock became standard
operating procedure for the space shuttle; by having a
portion of the vehicle that could be cordoned off and
taken to vacuum, the space shuttle could support a
two-person EVA while the rest of the crew remained
in a shirt-sleeves environment. The ISS operational
architecture also followed this approach, dedicating
an entire space shuttle mission – STS-104/7A in July,
2001 – to launching and installing the Quest Airlock
module, which was attached to the starboard side of
the Node 1 Unity module. The Quest Airlock made
ISS assembly and operations possible; due to the
large habitable volume of the ISS at assemblycomplete – approximately 965 cubic meters of
pressurized volume (and approximately 400 cubic
meters of habitable volume) – the lack of an airlock
would render EVA virtually infeasible [25]. In
comparison, the portion of the Quest Airlock required
to depressurize to vacuum for an EVA is a modest
8.8 cubic meters [6].
The value of possessing airlock capabilities was
not lost on mission designers when planning the
Constellation program’s return to the Moon. To
determine the value of an airlock, however, the
Exploration Systems Architecture Study (ESAS)
team conducted an airlock trade study [33].
Examining a number of airlock variable
permutations, the team finally concluded that:
In general, airlocks become more essential
as the number of ingress/egress cycles [i.e. EVAs]
increase. The ESAS team concluded that…for 7-day
sortie-class accessibility, an airlock is strongly
desired; and for an outpost mission, and airlock is
essential.
Figure 3 below shows a depiction of the Lunar
Sortie design reference mission for the Constellation
program.
Figure 3 – Constellation Program Lunar Sortie Design Reference Mission [33]
The Constellation program architecture’s
automation of systems eliminated the need for the
Command Module Pilot to remain aloft in low-lunar
orbit. For the lunar sortie missions (with a duration
on the order of 14 days), the crew complement would
be four astronauts, and upon arrival into low-lunar
orbit, all four would transfer from the Command
Module (CM) to the Lunar Surface Access Module
(LSAM), leaving the CM unmanned. The LSAM
would undock and descend to the surface, where the
crew of four would spend up to seven days
completing exploration objectives.
Based on the airlock trade study results, the
LSAM would include a bulkhead partition, to
segregate a portion of the habitable volume, which
could then act as an internal airlock. This would
allow the crew to don the Constellation space suits
and depressurize just that portion of the pressurized
volume to vacuum to facilitate egress. To maximize
the value of the lunar surface time, the entire crew of
four would have the capability to perform daily
EVAs, with all four crewmembers egressing from the
airlock together, and working on the surface in pairs.
In comparing a NEO mission to the above
programs, it is apparent that the unpredictability of
what constitutes the ideal target NEO requires a
robust design capable of supporting a wide variety of
mission profiles.
Crew complement will be
discussed in the following section, but regardless of
the number of astronauts flown to a near-Earth
object, the determination of the ESAS airlock trade
study team directly applies; the incorporation of an
airlock into the NEO exploration architecture is
essential and will provide invaluable flexibility.
5.
Crew Complement
The variety of possible destinations discussed
thus far demands flexibility in both operations and
hardware. By investigating a number of potential
target NEOs simultaneously through robotic precursor missions, the baseline design for the crewed
vehicle must be capable of supporting the mission
profile for any acceptable target NEO.
The
incorporation of an airlock into the crewed vehicle’s
design is one crucial way of increasing flexibility.
Another is the incorporation of operational
constraints for the crewmembers into the initial
architecture which directly affect the optimal crew
complement.
The EVA lessons learned from the construction
of the ISS led to an evolution of operational
constraints, termed Crew Scheduling Constraints
within NASA. Chief among these EVA operational
constraints was the number of EVAs that an
individual crewmember could perform over a given
period of time.
EVAs on the lunar surface consisted primarily of
walking and collecting samples, and eventually
traversing greater distances using the Lunar Rover
Vehicle (LRV). Suit design led to hand fatigue, but
by the time of Apollo 17, crewmembers were
routinely hiking up and working on the slopes of
craters, all the while maintaining relatively low
metabolic rates.
Aboard the ISS, however, complex microgravity EVAs demanded more of the crewmembers
physically. Fatigue led to inefficiencies, which led to
small mistakes.
The Shuttle Crew Scheduling
Constraints document was developed to capture these
lessons learned and formalize them into flight rules,
so that a guideline was set and all parties were in
agreement regarding the recommended best practices
for mission operations, including EVA.
The following is the excerpt from NASA’s
Shuttle Crew Scheduling Constraints [31], along with
the rationale of the constraint in italics. This
constraint applies for joint missions where the shuttle
is visiting the ISS, as well as shuttle-based EVAs
such as HST servicing:
•
A minimum of one FD [flight day] must separate
two scheduled EVAs for any given EVA
crewmember.
Rationale:
It is overly tiring for the EVA
crewmembers to perform an EVA two FDs in a row.
To perform consecutive EVAs, two teams of EVA
crewmembers must be available.
If an exception request is submitted against this
constraint, the following information must be
provided for the assessment: the duration of the
EVAs, details of the EVA tasks, details and crew
workload of other activities on the same mission, and
the identity of the EVA crewmembers. Preferably,
any request for back-to-back EVAs should be made
early enough in the planning process that the crew
office can select EVA crewmembers who are
experienced and best able to accomplish the
objectives.
In addition, if multiple EVAs are
required, then the back-to-back EVAs should not both
be physically intensive. In other words, try to
schedule the lighter workload EVAs as the back-toback sequence. More than two back-to-back EVAs
will not be schedule.
For stage increment EVAs aboard the ISS during
quiescent operations, NASA’s Generic Groundrules
Requirements and Constraints places even greater
limits on EVA operations [12]. The following is the
excerpt, as well as the rationale (again in italics):
•
For ISS EMU EVAs, the maximum number of
EVA cycles a crewmember should nominally be
scheduled for is 2 EVAs per week with at least 2
non-EVA days in between each EVA. However,
if, due to a mandatory Station assembly
requirement, the Station crew is required to
perform 3 EVAs in one week, then this will be
allowed.
Rationale: 2 EVAs can be scheduled per week for
multiple weeks up to the maximum number allowed
per crewmember and suit/airlock consumables (9 per
increment [assuming an increment is 180 days in
duration] - reference SSP 50261-01). Time is
required for crew rest and EVA replanning between
EVAs. However, it is recognized that there may be
an exception case where 3 EVAs are required to meet
a unique assembly requirement. In this case 3 EVAs
in one week are allowed.
These lessons were not lost on the Constellation
Program’s ESAS team. As stated earlier, sortie
missions to the surface of the Moon planned to allow
all four crewmember to conduct EVAs together each
day, for up to seven days. In the Executive
Summary, it simply states that:
For missions lasting beyond 4 days [on the lunar
surface], a rest day between EVAs may be required.
Although not as stringent as the wording of the
ISS and space shuttle constraints documents, it is also
not a groundrule but an indicator that daily surface
operations, even in the less physically demanding
environment of the lunar surface, is not advisable
[33].
Each of these constraints are applicable, in a
way, to a NEO mission, as a NEO mission is a blend
of them all. Proximity operations will more closely
resemble a shuttle mission, with a flurry of activity
over a few days to a few weeks. All surface
operations will occur during this condensed
timeframe, and the constraints learned from the
shuttle program are applicable in the very low-gravity
of a near-Earth object. However, the transit time to
the NEO makes the overall mission more closely
resemble an ISS increment, with the crew in the
micro-gravity environment of space for months. The
constraints for ISS quiescent operations is based on
the fact that the crewmembers have been on orbit for
an extended duration, and physical fitness is
degraded.
And surface operations themselves,
although in a very low-gravity environment, will
have many of the same objectives and challenges that
both the Apollo and Constellation programs faced.
Many of the studies thus far typically assume a
crew complement of two to three astronauts
[7,16,19]. The optimal crew complement is not an
arbitrary decision, however, and one of the greatest
flexibilities of including an airlock in the crewed
vehicle design goes hand-in-hand with the optimal
crew complement.
On space shuttle-based EVAs prior to the ISS
era, the maneuverability of the shuttle itself served as
a method of EVA crewmember rescue. While EVA
crewmembers utilized 55-foot long safety tethers
when working in the payload bay, the space shuttle’s
maneuverability was considered the second fault
tolerance, should a safety tether fail to keep a
crewmember attached to the vehicle. If an EVA
crewmember came free of structure and floated
“overboard,” the space shuttle Commander would
maneuver the orbiter into a position where the
drifting crewmember would float back into the
payload bay. In this way, the astronauts had a
secondary method of rescue.
ISS construction, however, required that the
space shuttle dock with the orbiting outpost, and the
ability to quickly retrieve a free-floating crewmember
with the orbiter was no longer possible. To solve this
fault-tolerance issue, NASA developed an emergency
jetpack, termed the Simplified Aid for EVA Rescue
(SAFER), to provide an untethered, free-floating
crewmember with a method of self-rescue. A
crewmember adrift would deploy a hand controller,
use the SAFER first to regain attitude control, and
then propel himself slowly back to the ISS.
For both the Apollo and Constellation programs,
this issue was of no concern, as the EVA crews were
held to the lunar surface under one-sixth-gravity.
This meant that the crewmembers exploring the
surface of the Moon need only be within walking
distance of the LM, should an emergency (a space
suit malfunction, for example) arise. Because of this,
and the advancements in the automation of systems,
the Constellation program had no need to maintain a
crewmember in the CM in lunar orbit; the CM would
never be used to rescue an EVA crewmember.
As was discussed earlier, however, near-Earth
objects rotate. That rotation, combined with the very
low-gravity, makes synchronous orbit dependent on
great quantities of propellant. Depending on the
chosen method of proximity operations at a target
NEO, a crewmember may be on the surface of a nearEarth object, out of view of an “orbiting” spacecraft.
The weak gravitation limits the mobility of an
astronaut – in particular, it limits the speed of an
astronaut’s mobility – and in an emergency such as a
space suit malfunction or a crewmember whose
efforts set him adrift from the NEO surface, rescue
may rely, as it did for space shuttle-based EVAs, on
the spacecraft itself.
For this reason – to maintain flexibility in the
methodology of proximity operations, allowing the
spacecraft itself to rescue a distressed EVA
crewmember – the spacecraft must at all times be
manned during EVAs. This precludes a mission
complement of two crewmembers, as EVAs are
always done in teams of two, and this would leave
the vehicle unmanned and incapable of performing a
crew rescue.
To examine the optimal configuration, taking
into account both the rescue capabilities of the
spacecraft and the operational constraints regarding
EVA scheduling, Table 3 delineates the permutations
of viable EVA crewmembers and airlock
configurations for a NEO mission with an arbitrarily
chosen ten-day proximity operations mission profile.
The maximum scheduled duration for an EVA will
be assumed to be six hours and thirty minutes; the
rationale for this will be discussed later.
Crew Comp/Airlock Capabilities
No. of Scheduling Violations
POD1
POD2
POD3
POD4
POD5
POD6
POD7
POD8
POD9
POD10
EVA Time Scheduled
Man-hours Scheduled
EVA Time Possible
Man-hours Possible
% of Possible Time Spent EVA
No. of Days w/o EVA
3Crew/2Person Airlock
0
1
EV1/EV2
EV1/EV2
OFF
EV1/EV3
EV2/EV3
EV2/EV3
OFF
OFF
EV1/EV3
EV1/EV2
OFF
EV2/EV3
EV1/EV2
OFF
OFF
EV1/EV3
EV2/EV3
OFF
OFF
EV1/EV2
32:30
45:30
65:00
91:00
65:00
65:00
130:00
130:00
50%
70%
5
3
4Crew/2Person Airlock
0
1
EV1/EV2
NA
EV3/EV4
NA
EV1/EV2
NA
EV3/EV4
NA
EV1/EV2
NA
EV3/EV4
NA
EV1/EV2
NA
EV3/EV4
NA
EV1/EV2
NA
EV3/EV4
NA
65:00
NA
130:00
NA
65:00
NA
130:00
NA
NA
100%
0
NA
4Crew/3Person Airlock
0
1
EV1/EV2/EV3 EV1/EV2/EV3
OFF
EV1/EV2/EV4
EV2/EV3/EV4
OFF
OFF
EV1/EV3/EV4
EV1/EV3/EV4 EV2/EV3/EV4
OFF
OFF
EV1/EV2/EV4 EV1/EV2/EV3
OFF
OFF
EV1/EV2/EV3 EV2/EV3/EV4
OFF
OFF
32:30
39:00
97:30
107:00
65:00
65:00
195:00
195:00
50%
60%
5
4
Table 3. Airlock and Crew Complement for a sample mission with 10 days spent performing proximity operations.
Each day at the NEO is identified as Proximity Operations Day X (POD 1 represents Proximity Operations Day 1,
the first full day after arriving at a NEO). EV# is the unique identifier for each specific EVA crewmember.
The above example for a mission with ten
days of proximity operations indicates that even
when allowing for a back-to-back scheduling
violation where applicable, no permutation for a
three- or four-crewmember complement can
match the capabilities of a four-crewmember
spacecraft with an airlock designed for two
people.
This configuration maximizes the
number of hours spent performing surface
operations (100% of the possible number of
EVA hours supportable by the airlock
configuration), while having no scheduling
violations, and no required days without an
EVA. In fact, in this configuration, there is no
need to even analyze scheduling violations,
because back-to-back EVAs conducted by a
single crewmember cannot add any time to the
number of hours spent performing surface
operations.
Of course, a crew larger than four opens up
many more permutations to improve the amount
of time spent performing surface operations, but
trade studies need to be conducted to determine
the benefit of an expanded crew complement
versus the changes to vehicle mass, required
consumables, and mission delta V, to name just a
few impacted parameters.
At the other end of the crew complement
spectrum, a mission with much more time at a
NEO target may desire days off, and in this case
a crew of three may be acceptable. Jones et al.
analyzed a crewed mission to 1991 VG in the
1991 to 1992 timeframe, when the asteroid was
just 0.004 AU from Earth. The mission profile
supported a 60 to 90 day mission, including 30
days performing proximity operations [17]. In
this scenario, a crew of three astronauts could
support all scientific endeavors with no impacts
to operational constraints, because the large
number of days performing proximity operations
lessens the impact of the required days off.
A significant impact to this such mission
profile (or any exploration architecture that
counts on a single crewmember being aboard the
spacecraft while the others conduct an EVA) that
should not be overlooked is with regard to using
the spacecraft itself as a method of crew rescue.
Since each crewmember would be aboard the
vehicle alone at some point, all three must be
fully capable of piloting the spacecraft to rescue
a stranded or injured EVA crewmember. This
would have an impact to pre-mission training
(and potentially astronaut selection itself), as a
geologist, an astrophysicist, an engineer or a
medical doctor (all career categories which
currently qualify for astronaut selection), would
need to be as adept at flying the spacecraft as the
military pilot(s) who would likely be the primary
pilots.
So while a three-person crew
complement is an option for NEO missions with
long-duration
proximity
operations,
its
inflexibility keeps it from being the optimal
choice for a NEO exploration architecture.
Therefore, to maintain the flexibility to
explore any NEO assessed by a precursor
mission, the nominal crew complement should
be no less than four astronauts, with an airlock
designed for use by a two-person EVA team.
This requires that only two crewmembers be
capable of piloting the spacecraft, and in fact
allows the remainder of the crew to be experts in
other fields. As was shown with Apollo 17 –
which flew the first scientifically trained Mission
Specialist – having geologists (or astrophysicists,
engineers or even medical doctors) provides each
mission with greater capability. Additionally, a
crew of four ensures that surface operations
opportunities are maximized, without violating
the lessons learned from past flight experience,
regardless of whether proximity operations last
three or 30 days.
6.
Surface Operations – An Overview
The reasons for exploring near-Earth objects
– to better understand the formation of the solar
system, to advance exploration technologies such
as radiation shielding to allow humans to travel
farther away from the Earth for longer periods of
time, and to develop the capabilities to mitigate
potentially hazardous asteroids (PHAs) – has
been extensively documented. What is lacking,
at this time, is an operational concept of the way
in which surface operations will be conducted,
and the tasks that will satisfy the above
exploration objectives.
One way to create a baseline of EVA
activities for near-Earth object exploration is to
examine the activities employed by the Apollo
program to explore the Moon, and then map
those activities to NEO exploration. This can be
done effectively in the following manner:
•
•
•
•
Categorize the EVA tasks conducted on the
Apollo missions that visited the lunar
surface.
Identify the challenges experienced by the
Apollo astronauts in conducting each EVA
task.
Map the lunar EVA tasks to the NEO
environment and identify the subsequent
challenges.
Propose ways to mitigate those NEO EVA
challenges, to meet the NEO exploration
objectives.
By utilizing the above methodology, it is
possible to create a generalized set of tasks that
can then be transposed, or mapped, to the
activities that would constitute the backbone of
NEO surface operations.
7.
Mapping EVA Competencies to a
NEO
The disparity between the first lunar EVA
on Apollo 11 and the three EVAs of the last
lunar mission, Apollo 17, is significant. Apollo
11 conducted a single EVA of 2 hour and 31
minutes, venturing out around the Lunar Module
(LM) to perform very basic tasks, such as taking
photos, collecting rock samples from the surface
and planting the American flag. Compare this to
Apollo 17, the first mission flown by a scientist
Mission Specialist (geologist Dr. Jack Schmitt).
The Apollo 17 mission conducted three EVAs
for a total of 22 hours and four minutes, and
utilized the Lunar Rover Vehicle (LRV) to
traverse approximately 11 km away from the LM
on both EVA2 and EVA3. In comparison to
Apollo 11, it is apparent that the EVA objectives
evolved from simply stepping upon the surface
of the Moon to performing true field geology.
And while the complexity grew with each
mission, much of the overarching exploration
methodology changed very little. In essence, the
exploration tasks can be grossly broken down
into the following categories:
•
•
•
•
•
•
•
EVA preparations (and crew day length)
Egress and ingress
Surface mobility
Scientific experimentation setup (Apollo
Lunar Surface Experiments Package
(ALSEP) flew on Apollos 12-17, and the
modified, lighter-weight Early Apollo
Scientific Experiments Package (EASEP)
flew on Apollo 11)
Field Geology
o Surface Sample Collection:
§ By hand
§ Using tools (tongs, scoop
and rake)
o Trench sample collection using the
long-handled scoop
o Core samples collection using core
tubes and the hammer
o Deep core sample collection using
the Apollo Lunar Surface Drill
(ALSD) (Apollo 15-17)
Geological traverse using the LRV (Apollo
15-17)
Photographic Documentation
Even though this paper focuses primarily on
mapping the Apollo lunar activities to a NEO
mission, it also incorporates the lessons learned
from orbital EVA experience, when those
lessons supersede the Apollo experience. The
distinct advantage of this approach is that
mission designers can use both the experience
gained from the Apollo missions and the vast
knowledge that has come from the orbital EVAs
conducted since then (especially from the
expertise achieved through the design and
completion of the complex EVA tasks needed for
ISS construction). This will be most obvious
when examining egress and ingress.
8.
EVA Preparation and Crew Day
Length
EVA preparation refers to the activities that
must be completed prior to the start of an EVA.
Depending on the hardware complement flown,
tools and space suits may require reconfiguration
between EVAs. The operational architecture for
a NEO mission should attempt to minimize the
amount of time spent between EVAs on
hardware reconfiguration, as it can be
exceedingly time consuming. If the architecture
desires daily EVA capabilities (with alternating
2-person teams, as suggested above), the amount
of reconfiguration time each evening after an
EVA – in preparation for an EVA the following
day – will directly impact the following day’s
operations, should the crew fall behind the
timeline. Thus, whenever practical, hardware
quantities (space suits, gloves critical spares,
etc.) and operational concepts should be chosen
to minimize the frequency of hardware
reconfigurations.
To limit the risk of Decompression Sickness
(DCS), an EVA preparation prebreathe protocol
should be adopted that efficiently purges the
body of excess nitrogen in the shortest
acceptable duration. For example, when the
vehicle’s atmospheric pressure is 1 Atm (14.7
pounds per square inch (psi)), the required
amount of time spent breathing pure oxygen to
purge the body of excess nitrogen is four hours.
Conversely, at an atmospheric pressure of 10.2
psi, the oxygen prebreathe duration drops to 40
minutes.
Reducing the vehicle’s nominal
atmospheric pressure even further would reduce
the prebreathe duration, but would make the
vehicle
more
susceptible
to
critical
depressurization caused by, for example, a
micrometeoroid strike, so a trade study would
need to be conducted to define the optimal
vehicle atmospheric pressure.
As with all space programs thus far, and
because of its relative proximity to the Earth’s
orbit, a NEO mission will likely operate on a 24hour clock, to remain in synch with the ground
team. In doing so, the amount of time available
each day for scheduled activities can be assumed
to be a blend of the scheduling constraints of the
space shuttle and ISS programs, where it more
closely resembles an ISS mission profile during
transit to and from a near-Earth object, and
resembles a space shuttle mission profile – or
even more likely, a space shuttle EVA day
profile – throughout NEO proximity operations.
Table 4 shows the breakdown of a nominal crew
day for both the space shuttle and ISS programs,
and an EVA day for a space shuttle mission.
Activity
(Hrs)
Sleep
Post-Sleep
Midday
Meal
Exercise
Active
Duty
Pre-Sleep
ISS
8.5
2
1
Space
Shuttle
8.0
3
1
Space Shuttle
EVA Day
8.0
2.5
NA
2.5
6.5
1.5
8.0
2
3.0
NA
11.0
• 3.0
Hrs
EVA Prep
• 6.5
Hrs
EVA
• 1.5
Hrs
EVA
Cleanup
2.5
Table 4. Crew Day Length for ISS
and Space Shuttle Programs [12,31]
The Shuttle Crew Scheduling Constraints
provides the rationale for EVA duration [31]:
•
For scheduled and unscheduled EVAs, the
planned EVA PET shall not exceed one 6hour and 30-minute period per day.
Rationale: EVA duration is limited by an EVA
crewmember’s physical stress, by EMU
consumables, and by the crew workday length.
EMU primary life support system consumables
limit is 7 hours (Reference STS Operational
Flight Rules Book All Flights, rule A15.1.1-1 for
EMU time limits). Of this 7 hours, 30 minutes is
for primary consumables reserve, leaving 6.5
hours for EVA tasks. Oxygen and water supplies
can be recharged at vacuum, but the EMU
batteries and Lithium Hydroxide (LiOH)
canisters cannot. After 6.5 hours of PET the
crewmembers will be tired and may experience
cold extremities due to reduced circulation in the
suit. In addition, because of all the required preand post-EVA activities and the length of the
EVA itself, the crew workday will near the
maximum limits and should not be extended.
With the minimum EMU prebreathe (40
minutes), the minimum required pre- and postEVA activities, and a 6.5 EVA PET, the crew
workday is approximately 11 hours.
The
standard workday length is 10 hours. Because
of the above factors, when using a 10.2 Pounds
Per Square Inch (psi) prebreathe protocol,
scheduled EVAs are limited to 6.5 hours PET.
Experience has shown that crew fatigue can
lead to errors, and thus the amount of time the
crew can work each day is limited.
It is
therefore prudent to apply the lessons learned
from the ISS and space shuttle programs
regarding EVA duration and crew day length.
For proximity operations, the crew day length
should be limited to 11 hours, with a nominal
EVA duration limit of six hours and thirty
minutes.
9.
Airlock Egress and Ingress
The official start time of an EVA aboard the
space shuttle or ISS occurs when the first EV
crewmember transitions from vehicle power to
internal EMU battery power. Within minutes of
this transition, the crew completes airlock
depressurization and opens the EV hatch.
Airlock egress and ingress are thus part of the
nominal EVA tasks, and the duration of each
needs to be included in the EVA planning, to
ensure the 6.5 hour nominal EVA duration is not
exceeded. Therefore, even though the various
operational concepts may dictate different
methods of crew placement onto the NEO
surface, translating away from and back to the
spacecraft will be considered part of the egress
or ingress operations, and thus part of the
nominal EVA timeline.
The most significant difference between
Apollo and ISS EVAs, with respect to airlock
egress and ingress, is the effect of gravity on
methodology.
Upon opening the hatch on
Apollo 11, Armstrong crawled out onto the
porch on his hands and knees. His first task was
to release the Modular Equipment Stowage
Assembly (MESA), which held the sampling
tools and sample return containers (SRC), along
with the black-and-white camera that would be
used to film Armstrong as he stepped onto the
surface of the Moon for the first time. He
descended the LM ladder to the footpad and
using his toe, tested the soil to ensure it was solid
enough to bear his weight. When he was
confident he would not sink into feet of lunar
dust, he made one small step.
An astronaut aboard the ISS does not have
as simple a task when leaving the safety of the
airlock. The one-sixth gravity of the Moon
allowed Armstrong to egress and descend to the
surface unencumbered. In the micro-gravity of
space, however, an astronaut must utilize tethers
to restrain both himself and his tools, lest he find
either adrift from the spacecraft.
Prior to opening the hatch aboard the ISS,
both crewmembers in the airlock employ a 36inch long, fixed length tether to connect the
EMU to an anchor point inside the airlock.
Termed a waist tether because it connects to a Dring on the waist of the EMU, the waist tether
has a hook at each end and is used to ensure that
a crewmember does not drift away from structure
should he let go with both hands to complete a
task. Once the hatch is opened, the first
crewmember egresses the airlock. Once outside,
he attaches the anchor hook of another tether,
this one termed a safety tether, to an anchor point
on the outside of the airlock. This safety tether
is also attached to the EMU D-ring, but rather
than being a fixed length, this tether is 85 feet
long and made of a 3/32” braided stainless steel
cable. The cable is wound around a spool in an
enclosed reel, and is designed to tend out as the
crewmember translates away from the anchor
point, and retract (with a very low spring force)
as the crewmember translates back toward the
anchor point. In this way, the crewmember is
always attached via a cable to the spacecraft,
such that should he become physically separated
from structure, he still has one life-line to keep
him from becoming a human satellite.
Herein lies the drastic difference between a
lunar EVA and an orbital EVA; with even onesixth gravity, an Apollo astronaut did not need to
worry about floating away from the lunar
surface. He was able to translate on the surface
without being tied to the LM, and in later
missions it gave the Apollo astronauts the
freedom to travel and explore many kilometers
from the LM.
For a NEO mission, the
gravitational field will be stronger than the
micro-gravity of space, but tethering protocols
will still be required.
Depending on the
gravitation of a target NEO, it may even be
possible for an astronaut to impart enough force
to reach escape velocity. Therefore, a NEO
astronaut will need to remain tethered at all
times. This adds considerable complexity to a
NEO EVA over an Apollo EVA, and may even
play a more restricting role than aboard the ISS.
9.1 Tether Protocol – NEO “Landing”
If the spacecraft is capable of “landing” and
grappling onto the NEO’s surface, the
crewmembers could use a tethering protocol
similar to that used aboard the ISS, where the
safety tether is anchored directly to the
spacecraft. This would allow the astronauts to
maintain a lifeline to the spacecraft, ensuring the
ability to quickly return to the airlock in a
contingency scenario that required an expedited
ingress.
9.2 Tether Protocol – NEO “Mooring”
If the spacecraft can moor to the NEO and
remain directly overhead, the crew can employ a
similar tether protocol, but instead of tethering to
the spacecraft, the astronauts would tether to the
mooring line. The space shuttle and ISS both
utilized slide wires to aid in translation about the
respective vehicles. By attaching the anchor
hook of the safety tether to the slide wire, the
tether gained a degree of freedom; the anchor
hook was able to slide along the length of the
slide wire, providing extended range. In the case
of the space shuttle, the slide wire ran the length
of the payload bay and gave a crewmember the
ability to translate the payload bay’s full length
without extending the safety tether.
If a mooring line is employed at a NEO, the
slide wire concept can be transposed to the
mooring line. Upon airlock egress, an astronaut
would attach his safety tether to a slider on the
mooring line and descend under his own power
along the mooring line to the surface of the
NEO.
This poses advantages and disadvantages.
The primary advantage mirrors that of being
tethered directly to the spacecraft: in an
emergency, an astronaut has a lifeline (made up,
in this case, of the safety tether to the mooring
line, and the mooring line to the spacecraft) to
follow back to the airlock. Additionally, as
mentioned earlier, any mooring line has the
potential to break free of the surface. Should this
happen, the crewmembers are tethered to the
mooring line, ensuring that the crewmembers
possess the lifeline to the spacecraft even if the
spacecraft is no longer in contact with the
surface.
The obvious disadvantage is that the
crewmembers are tethered to the mooring line
should it break free of the surface. If the
spacecraft drifts from its position after a failure
of the mooring anchor, it has the potential to pull
the crewmembers across the surface of the NEO,
which could be detrimental to the pressurized
spacesuits, or it could pull the crewmembers
completely off the surface, which could cause
the astronauts to tumble and become wrapped in
their safety tethers. Adrift in free space, tangled
in a safety tether, an astronaut would be in a
precarious position without the aid of his EV
partner.
Due to the great risk of being dragged across
the surface of a near-Earth object covered in
jagged rocks, this method is not recommended.
If the spacecraft’s proximity operations dictate a
mooring method, it is advisable to devise a
method to decouple the crewmembers’ safety
tethers from the mooring line by a separate,
crewmember-installed anchor that is set into the
NEO itself once the astronauts reach the surface.
The crewmembers can then swap the safety
tether anchor point from the mooring line to this
newly-installed safety tether surface anchor.
Such an anchoring system has its own
inherent risks, however, that cannot be
overlooked. Should the mooring line break free,
there is no direct path to the spacecraft’s airlock
for an astronaut in an emergency. This would
likely dictate that the crewmembers be outfitted
with an emergency mobility unit similar to the
Simplified Aid for EVA Rescue (SAFER) that is
employed aboard the ISS. The SAFER mounts
to the bottom of the EMU PLSS and has a
deployable hand controller module (HCM) to
allow the crewmember to steer himself back to
structure if circumstances put him in free space.
This scenario would be less likely, however,
as it is two failures deep. First it is dependent on
a failure of the mooring anchor, and then
simultaneously on a suit or hardware failure that
requires an expedited ingress.
Should the
mooring line anchor fail, the crewmembers
aboard the spacecraft would nominally act to set
the mooring line again, and at the completion of
the EVA, the EV crewmembers would translate
to the reset mooring line and ascend to the
airlock.
9.3 Tether Protocol – NEO “High Hover”
In a high hover proximity operations
concept, the spacecraft cannot “land” or moor to
the surface of the NEO, and must instead simply
position itself in proximity to the NEO. During
airlock depressurization, the spacecraft would
lower its relative altitude to approach the NEO,
and upon egress, the crewmembers would have
to traverse across free space to the surface using
a propulsive mobility unit.
This operational concept provides the
greatest flexibility regarding the choice of
targets; by eliminating the need for the vehicle to
come into contact with the surface, the size,
shape, physical composition and rotation rate
have less influence over proximity operations in
general terms. Varying gravitational fields and
overall shape will dictate how close the
spacecraft can approach the surface, but with the
capability to traverse free space, the EV
crewmembers
provide
human-in-the-loop
flexibility to
viable
proximity-operation
methodologies.
The greatest challenge for the EV
crewmembers will be descending to the surface
with an angular velocity that matches the NEO’s
rotation rate.
Without precisely matching
angular velocities, the EV crewmember will
either approach too slowly or too quickly,
resulting in the relative motion making the
crewmember feel as though he is skidding across
the surface. Because of the limited weathering
of the NEO surface, there is great risk of damage
to the spacesuit from jagged rocks strewn across
the surface of the NEO.
This can be mitigated by flying the
spacecraft in a synchronous orbit of the NEO
during egress, so that the initial motion of the EV
crew is in synchronicity with the surface. A
crewmember can then use the propulsive
mobility unit to descend to the surface, tweaking
the angular velocity to settle onto the surface
with zero relative angular velocity.
This
operational concept would require two
constraints be met. First, from a propulsion
perspective, the vehicle would have to be
capable of flying multiple synchronous orbits, to
allow as many EVAs to be performed as possible
for a specific mission profile. Second, the
propulsive mobility unit used by the EV
crewmembers would have to be significantly
more capable than the SAFER. The SAFER was
designed for contingency use only, and is
propellant limited. A propulsive mobility unit,
used as described above, would be employed as
part of the nominal operations, and thus would
need a capability (and thus a propellant storage
and refueling system) significantly greater than
the SAFER, to ensure nominal usage over the
course of an EVA, with reserves for contingency
operations.
Similarly, by selecting a target NEO with a
single axis of rotation, a polar approach can limit
the effects of rotation rate on crewmembers as
they descend to the surface. Positioned over the
pole, the crewmembers can traverse free space in
the same manner as above, but the smaller
starting relative angular velocity will place less
demand on the propulsive mobility unit as the
crewmember approaches the surface with zero
relative angular velocity.
The greatest
disadvantage of utilizing a polar approach profile
is that it limits the exploration area to that around
the pole, making areas of interest near, say, the
equator, much more difficult to investigate.
The distinct disadvantage to relying
primarily on a propulsive mobility unit –
whether it be simply to traverse to the surface, or
to act as the primary mode of surface mobility –
is the way in which it will interact with the NEO
environment. Typically such systems employ
cold-gas thrusters, oriented along all three axes
to provide not only propulsion, but attitude
control. Should the surface of a target NEO be
inundated with dust, the plume of cold-gas
thrusters could create white-out conditions,
similar to disturbing the silt while scuba diving.
With very low-gravity, the dust will not quickly
settle, and repeated jet firings could exacerbate
the problem, possibly making it impossible for
an astronaut to maintain situational or spatial
awareness. The information gained from a
robotic precursor mission will help classify the
dust content, but building an exploration
architecture around thrusters as the primary
mode of mobility may severely limit the number
of acceptable destinations.
9.4 Open Work
The wide variety of near-Earth objects
dictates that any single method is not likely to
support mission operations at all targets of
interest. One way to combine “landing” and
“mooring” methods together, for example, may
be to use a variable length mooring line: whether
using a physical or chemical anchoring system,
the variable length line could be retracted to
place the vehicle onto the surface of the NEO, if
desired, or allow it to reel out and remain farther
from the surface. As the ability to classify nearEarth object surface composition improves (from
land-based or space-based observatories, or from
robotic scout missions), anchoring methods can
be chosen and optimized, but until there is more
hard data, a slew of techniques based off loose
assumptions will need to be developed, tested
and added to the toolbox of capabilities for NEO
exploration.
exploration. Additionally, there has been a
considerable amount of work done to
characterize Phobos – mean diameter, rotation
rate, and bulk density are all measured quantities
– making an analysis more accurate. Figure 4 is
an image of Phobos taken from NASA’s Mars
Reconnaissance Orbiter on March 23, 2008 [20].
10. Surface Mobility
One of the greatest challenges to low-gravity
body exploration will be to develop ways to
translate across the surfaces of the various
bodies, each with its own unique gravitational
field. The largest NEO, 1036 Ganymed, has a
diameter of 31.66 km, and it is significantly
larger than the next largest NEO, 433 Eros,
which has a mean diameter of 16.84 km.
Compare the size of these objects to 2010 RF12,
which has an estimated diameter of six to 14
meters, and passed within approximately 79,000
kilometers of Earth in September of 2010, and
one can see the vast array of objects which exist
under the NEO moniker [38].
Due to this great disparity in the size of
potential targets, an exploration architecture
must possess the capabilities to explore a lowgravity body with a diameter, for example, of
just 200 meters, and one as big as 1036
Ganymed, in an equally effective manner. Add
in main-belt asteroids and the moons of the gas
giants to the low-gravity body population, and
the list of potential targets is nearly limitless.
As mentioned earlier, a potential target may
be attractive because of its size, composition,
distance from Earth, rotation rate, or various
other parameters. Yet this vast population, along
with the various classes of these objects, implies
the inability to create a “one-size-fits-all”
exploration architecture for very low-gravity
bodies.
A starting point, however, must be chosen.
For the purposes of this paper, the potential
target of choice represents a viable mission
target, even if the exploration architecture of the
future chooses to steer away from near-Earth
objects. Because Mars will always remain a
high-value target, this paper will use Phobos, one
of the two Martian satellites, as the
representation of a very low-gravity body for a
surface mobility analysis.
Phobos is larger than all but one NEO, but
easily fits within the range of very low-gravity
bodies that will require new EVA tactics for
Figure 4. MRO image of Phobos[20]
10.1 Phobos Properties and Assumptions
This paper will model Phobos as a sphere
with constant bulk density, rotating about a
single axis. This allows a simpler calculation of
the gravitational acceleration at the surface.
Inconsistent bulk densities or non-spherical
shapes alter the site-specific surface acceleration
due to gravity, introducing further challenges to
an astronaut attempting to traverse across the
surface. Table 5 presents the parameters of
Phobos relevant for this analysis.
Parameter
Gravitational
Parameter
Bulk
Density
Porosity
Mean
Radius
Period
Symbol
GM
ρ
Value
0.7127 ± 0.0021 x
10-3 km3/s2 [4]
1876 ± 20 kg/m3 [4]
R
30% ± 5% [4]
11.1 ± 0.15 km [37]
P
0.3189 days [37]
Table 5. Phobos: given properties (note that G represents the
gravitational constant and equals 6.6725 x 10-20 km3/kg/ s2)
The gravitational acceleration will then be
calculated by:
=−
(1)
yielding a surface gravitation value of 5.784 x
10-3 m/s2 acting toward the center of Phobos. To
calculate the total surface acceleration, however,
the gravitational acceleration must be summed
with the centripetal acceleration caused by the
rotation rate, acting opposite the gravitational
acceleration.
=(
) −
(2)
where ω is the rotation rate, in rad/s, and is the
latitude (measured from the equator). The
greatest value for
(and thus the greatest
effect that centripetal acceleration will have on
the total sensed gravitation) will be when = 0.
Knowing the period, ω can be easily calculated.
Equation 2 produces gTOT = 5.207 x 10-3 m/s2.,
directed toward the center of Phobos. This is the
gravitational acceleration the astronaut will sense
when working on the surface at the equator [30].
10.2 Surface Mobility: Proper Orientation
Astronauts of the Apollo program adapted to
one-sixth gravity quickly, finding it easy to
develop a method of loping across the surface of
the Moon. For an astronaut on the surface of a
very low-gravity body, like Phobos, translation
will not be so simple. To calculate the optimal
walking speed on the surface of a body, the
Froude number (Fr) is utilized:
=
control system would be necessary to maintain a
controllable upright posture.
In light of the difficulties associated with
walking, an important question to ask is if
walking is even the most desirable method of
mobility? If the purpose of exploring the surface
of Phobos is to perform field geology and collect
surface samples, then the answer is likely no.
The rationale for not employing significant
resources to solve the challenges of walking in a
very low-gravity environment was unwittingly
given by Neil Armstrong during the Apollo 11
mission debrief [15].
When talking about
surface operations and the optimal way to work,
Armstrong said:
In general, there were a lot of times that I
wanted to get down closer to the surface for
one reason or another. I wanted to get my
hands down to the surface to pick up
something.
This was one thing that
restricted us more than we’d like…We
should clear the suit so that you could go
down to your knees, and we should work
more on being able to do things on the
surface with your hands. That will make our
time a lot more productive, and we will be
less concerned about little inadvertent
things that happen.
Therefore, to simplify surface exploration
and to better meet the exploration objectives, the
preferred body orientation would be horizontal,
more or less parallel to the surface, and within
arms’ reach of the surface.
(3)
where v is the speed of movement (in m/s), g is
the total gravitation sensed at the surface (in
m/s2) and l is the leg length (in meters). For an
average human male, l = 0.92 m. Additionally,
Fr is about 0.25 for optimal walking speed, and
about 0.5 for the walk-to-run transition speed
[26].
Using these values, the optimal walking
speed and walk-to-run transition speed for an
astronaut on the surface of Phobos is 0.035 m/s
and 0.049 m/s, respectively. When putting this
into terms easier to visualize, these speeds are on
the order of one-tenth of one mile per hour. At
speeds greater than this, the astronaut will begin
very long, parabolic trajectories that will be
difficult to control. Therefore, if an operational
concept dictated that it was necessary for an
astronaut to walk on the surface of a very lowgravity body, a restraint system or attitude
10.3 Spacesuit Properties and Assumptions
The prone position affords a lower center of
gravity, greater stability and a much more
efficient body orientation for geological sample
collection. The primary mode of mobility will
not be the legs, but the arms. In this orientation,
an astronaut would be able to hop across the
surface of a very low-gravity body with much
greater ease than any attempt to walk would
allow. To understand the effect that this mode of
mobility will have on an astronaut, however, it is
necessary to calculate the motion as the astronaut
propels him or herself using just one’s arms.
To determine the motion, this paper will
employ an estimated model of the Extravehicular
Mobility Unit (EMU) currently used for EVA
aboard the ISS. Figure 5 provides a gross
estimation of the EMU geometry relative to the
center of gravity (CG) [36]. Note that hCG
represents the height of the CG above the
surface. To calculate the motion, it will be
assumed that the astronaut can be represented as
a rigid body. This is not an entirely accurate
assumption, but simplifies the analysis of the
motion.
Under even very low gravitation, the
positioning of the astronaut above the surface, as
shown in Figure 5, is obviously not a stable one.
With gravity pulling down, it is easy to envision
that an astronaut will always eventually settle
into a position where he or she is laying facedown on the surface. This analysis, however,
assumes that this prone position is in fact stable;
the way in which the astronaut physically
maintains this prone position above the surface
will be discussed later.
L
0.625L
0.375L
0.125L
Y
Z
X
0.07L
hCG
Surface of Phobos
Figure 5. EMU estimated center of gravity geometry
(The coordinate frame represents the EMU itself and
not the local-vertical / local-horizontal coordinates
at the surface of Phobos)
Table 6 provides the above measurements,
along with a measurement for the estimated
length of a fully-extended arm. Note that these
estimated measurements are applicable only for
the illustration that follows. A precise evaluation
of the motion of the EMU (or any future
spacesuit designed for exploration) would
require precise measurements relative to the
center of gravity, along with a precise
measurement of the moment of inertia (I).
Segment
CG-Foot
CG-Shoulder
(Horizontal)
CG-Shoulder
(Vertical)
CG-Head
ShoulderPalm
CG-Palm
Representative
Distance from
CG
0.625L
0.125L
Distance
to
CG for L=2.0
meters
1.25 meters
0.25 meters
0.070L
0.14 meters
0.375L
0.275L
0.75 meters
0.55 meters
0.345L
0.69 meters
Table 6. EMU estimated center of gravity geometry
For the purposes of this analysis, it will be
assumed that the total mass (m) of the EMUplus-crewmember is m = 300 kg, and the
moment of inertia about the y-axis will be taken
to be Iyy = 50 kg m2, a number within the range
of values measured on orbit as part of the testing
of the Simplified Aid for EVA Rescue (SAFER)
on STS-64 in September, 1994 [36].
Finally, to be able to examine the
translational and rotational motion of the
spacesuit on a very low-gravity body, it is
necessary to know how much force an astronaut
is capable of imparting. Per NASA’s ManSystems Integration Standards, the forces that a
free-floating crewmember (one not held rigidly
in place by a restraint) can impart are as follows
in Table 7 [14].
Linear Force
4.4 N (1.0 lbf)
22.2 N (5.0 lbf)
44.5 N (10.0 lbf)
Duration
4.5 sec
2.1 sec
1.4 sec
Table 7. Maximum forces and duration capable of
a free-floating crewmember [14]
10.4 Translational/Rotational Motion Analysis
The motion of the astronaut away from the
surface must be calculated in two parts. First,
while the crewmember is applying the force
normal to the surface, the distances, velocities
and accelerations due to the applied force can be
calculated. Then, when the crewmember is in
free-flight (meaning under only the net
gravitational
acceleration,
gTOT),
those
translational and rotational motions can be
determined. Figure 6 shows the free-body
diagram of the forces acting on the astronaut.
While an astronaut would want to apply a force
that creates not only upward motion, but forward
motion as well, a force in only the +h direction
simplifies the calculations and is sufficient to
illustrate the rotational motions. Note that the
applied force generates a counter-clockwise
(negative) rotation.
+h
The motion for each applied force is listed in
Table 8.
F
(N)
4.4
22.2
44.5
F
+ω
τ
tpush
(s)
4.5
2.1
1.4
at
(m/s2)
0.015
0.074
0.148
In a similar manner, the rotational motion
due to the push-off can also be calculated, using
Equations 7-9.
mg
Surface of Phobos
Figure 6. EMU free-body diagram
=
10.5 Motion During Normal Force Application
Using the mass value above, the
translational motion experienced during the
application of the forces in Table 7 (from this
point forward termed the push-off) can be
calculated using Equations 4-6.
ℎ
=ℎ
+
=
+
0.152+ hCGo
0.163+ hCGo
0.145+ hCGo
v
(m/s)
0.068
0.155
0.207
Table 8. Translational motion and arm extension length
θ
=
hCG (m)
(4)
+
(5)
(6)
The term at in Equation 4 represents the
translational acceleration normal to the surface,
in m/s2.
In Equation 5, the term hCG, in meters, is the
normal translational height above the surface of
Phobos that a crewmember travels by extending
his or her arms to their full length, just as one’s
hands are leaving the surface. As previously
mentioned, the force is applied normal to the
surface of Phobos, thus producing translational
motion in the +/-h direction.
To calculate just the arm extension length,
take the initial height of the CG, hCGo = 0 meters.
The term
is the initial vertical velocity, in
this case
= 0 m/s, and tpush is the time over
which the force is applied, in seconds.
In Equation 6,
is the positive vertical
translational velocity at the end of the push-off,
just as the crewmember’s hands are leaving the
surface. The initial vertical velocity,
, is zero.
=
=
= α
(7)
+
(8)
+
(9)
The term α in Equation 7 is the angular
acceleration, in rad/ s2, measured positive in the
clockwise direction, and r, in meters, is the
distance from the applied force to the center of
gravity. Per Table 2, the horizontal distance
from the CG to the rotation point of the shoulder
is r = 0.25 meters. In Equation 8, θpush, in
radians, measured positive in the clockwise
direction, is angle of rotation experienced by the
astronaut while applying the force normal to the
surface of Phobos, and θo is the initial rotational
displacement. For a crewmember in the prone
position, θo = 0 radians (i.e., the datum from
which θo is measured is parallel to the surface).
The term ωpush in Equation 9 is the angular
velocity of the astronaut’s rotation, in rad/s,
measured positive in the clockwise direction.
Again, since the astronaut starts this motion at
rest, the value for the initial angular velocity, ωo
= 0 rad/s. The solutions for the rotational motion
due to the push-off are listed in Table 9.
F
(N)
4.4
22.2
44.5
tpush
(s)
4.5
2.1
1.4
at
(m/s2)
0.015
0.074
0.148
α
(rad/s2)
-0.022
-0.111
-0.222
θpush
(rad)
-0.223
-0.245
-0.218
ωpush
(rad/s)
-0.099
-0.233
-0.311
Table 9. Normal acceleration and arm extension length
The most notable result from Tables 8 and 9
are the similarities between the values of hCG for
the translational motion, and the values of θpush
for rotational motion. This shows that the
varying forces applied by an astronaut result in
very similar initial motion. Since this is the case,
the remainder of this translational and rotational
analysis will be completed using the values
associated with an input force of F = 22.2 N.
10.6 Free-Flight Translational Motion
The rotational motion will have a
considerable effect on the orientation of the
astronaut, but the vertical translation of the CG is
unaffected by rotation and can be calculated
independently. With the linear velocity upward
(
)
already known,
the maximum
translational height of the CG, and the time it
takes to reach that point, can be calculated from
Equations 10 and 11:
=0=
(ℎ )
=ℎ
=
+
−
(10)
−
(11)
where tff , in seconds, is the free-flight time to the
maximum height of the CG, (hCG)Max,, measured
in meters. To get the total time to (hCG)Max, sum
the values of tff and tpush . Table 10 presents the
maximum vertical height and the duration for the
positive vertical translational motion.
F
(N)
4.4
22.2
44.5
tpush
(s)
4.5
2.1
1.4
tff
(s)
12.68
29.84
39.88
tCGmax
(s)
17.18
31.94
41.28
(hCG)Max
(m)
0.92
2.83
4.64
Table 10. Time and distance to CG maximum height
As one would expect, a smaller force
applied over a longer time period produces a
smaller vertical hop, lasting less time. An
increase in the gravitation sensed by the
crewmember would decrease both the flight time
and the height, as one would expect. Yet this
only describes the positive vertical translational
motion of the crewmember. The rotational
motion becomes more complex.
10.7 Free-Flight Rotational Motion
As shown, a constant force applied normal
to the surface generates positive vertical
translational motion. However, since the force is
not applied at the CG, it creates a counterclockwise rotation about the CG as well. It
stands to reason, then, that to avoid any rotation
in this case, one may move the CG to be
coincident with the shoulder. However, as an
astronaut will want to move both upward and
forward while exploring a very low-gravity
body, there is no way to place the CG such that
the horizontal and vertical components of such a
force are directed through the CG. Therefore, in
this orientation, any force applied by the
astronaut will create a rotation.
To understand the motion as the astronaut
moves upward and rotates counter-clockwise,
however, the analysis must be able to determine
when and if the feet strike the surface.
It turns out that, for any given applied force,
this is dependent on hCGo, the initial height of the
CG above the surface, measured in meters. Due
to the estimated geometry of the EMU, the
minimum value of hCGo cannot be less than 0.28
meters. At that initial height, the crewmember is
lying in contact with the surface. Additionally,
the maximum value of hCGo cannot be greater
than the arm extension length subtracted from
the CG-to-palm length of 0.69 meters. Table 11
shows the minimum and maximum initial CG
height for each input force.
F
(N)
4.4
22.2
44.5
Arm ext.
length (m)
0.152
0.163
0.145
(hCGo)Min
(m)
0.28
0.28
0.28
(hCGo)Max
(m)
0.538
0.527
0.545
Table 11. Minimum and maximum initial CG height
It is unrealistic to explore the surface of a
very low-gravity body while actually lying on
the surface, so for the first example, assume the
crewmember is positioned just above the surface.
Choose hCGo = 0.35 meters; this means that in the
prone position, the front of the EMU is just 7
centimeters from the surface. Knowing this
value for hCGo, one can determine if the feet
come into contact with the surface, and if so, at
what time (t) and at what angle (θ), using the
geometry from Figure 7 and Equations 12-17
below:
0.75m
EMU Position
after time t
hHead
X
hCG
-θ
Z
Y
hCGo
hFeet
EMU Initial
Position
Surface of Phobos
Figure 7. EMU geometric analysis
10.8 Rotational Motion: Altering hCGo
=
(12)
ℎ
=
sin ( )
(13)
= 1.25 −
ℎ
ℎ
=
=ℎ
(14)
( )
(15)
−
(16)
= ( + 0.75)sin ( )
(17)
where θ is in radians, and X, Y, Z, hFeet, and hHead
are measured in meters. When the value for
either hFeet or hHead is zero, that part of the EMU
is in contact with the ground. It should be noted
that the starting point of this free-flight rotational
motion coincides with the end of the push-off, as
this is the second part of the rotational motion
analysis.
Using the geometry above, and
knowing the applied force (F), the duration of
the applied force (tpush), and the initial height of
the CG (hCGo), it is possible to iterate upon the
free-flight time (tff) to determine if and when the
feet strike the surface during the positive vertical
translation. Table 12 displays the results for hCGo
= 0.35 meters.
F
(N)
4.4
22.2
44.5
Time (t) when
hFeet,=0
8.5 sec
4.1 sec
3.0 sec
sufficient input force, a relationship exists that
may cause the positive translation of the CG to
outpace the rotation counter-clockwise, thus
keeping the feet from ever touching the surface
during the positive vertical translation. This
gives rise to two other questions. First, rather
than increase the input force further, does a value
exist within the allowable range of hCGo that
causes the positive translation of the CG to
outpace the rotation, allowing unimpeded
rotation throughout the entire positive vertical
translation? Second, what is the continuing
motion of the EMU throughout a particular
trajectory? These two topics will be addressed
serially in the next two sections.
hCG
(m)
0.723
0.809
0.821
θ
(rad)
-0.617
-0.704
-0.716
Table 12. EMU position when feet contact surface
Table 12 shows that increasing the force
causes the feet to strike the surface sooner, but at
a greater angle. This implies that, given a
By varying hCGo and performing the same
analysis as before, it is possible to determine
whether or not a lower limit of hCGo exists,
within the allowable range for each applied
force, that results in unimpeded rotation
throughout the positive vertical translation. This
threshold limit will be termed (hCGo)T.
For an input force of 4.4 N, no allowable
value of hCGo produces unimpeded rotation; the
astronaut’s feet always contact the surface during
the positive vertical translation. At (hCGo.)Max,
the feet contact the surface at t = 11.89 seconds,
with a CG height of 1.020 meters, and a rotation
of 0.954 radians counter-clockwise.
At this applied force, an astronaut would
never propel him or herself into an unimpeded
rotation, where one is unable to use one’s legs to
dampen rotational motion. What is unknown is
how effectively an astronaut can use his or her
legs to dampen out such a motion. Also note
that these numbers apply only to the gravitation
of Phobos. If an astronaut was exploring any
other very low-gravity body, with its own
unique gravitational field, this analysis would
need to be repeated for said gravitation.
When the force applied is equal to 22.2 N, a
value for (hCGo)T does exist where the astronaut’s
feet do not contact the surface during the positive
vertical translation.
Instead, the astronaut
rotates, unimpeded, counter-clockwise for the
entire trajectory, not making initial contact with
the surface until he is descending toward it
approximately 59 seconds later. Unimpeded
rotational motion occurs for any value of hCGo =
0.416 meters or greater, up to (hCGo.)Max Figure 8
depicts the astronaut’s motion with hCGo =
(hCGo)T = 0.416 meters.
The closest the
astronaut’s feet come to the surface is a single
millimeter, when θ = -0.918 radians
t = 5.70 sec
θ = -1.084 rad
+h
+ω
hCG = 1.105 m
Surface of Phobos
Figure 8. No contact motion, F=22.2N, hCGo=0.416 m
A value for (hCGo)T also exists for F = 44.5
N. For this applied force, no contact will be
made with the surface for any value of hCGo =
0.403 meters or greater. In this case, at t = 4.1
seconds, the astronaut’s feet are just two
millimeters from the surface. This occurs when
θ = -1.059 radians, and hCG = 1.091 meters.
From this information, a relationship can be
discerned between the applied force and the
value of hCGo for which the rotational motion is
unimpeded throughout the positive vertical
translation. To more accurately define this
relationship, however, more data points for the
maximum forces a free-floating astronaut can
impart (along with the duration of that force
application) would be needed.
Yet from even these few data points, the
relationship shows that as the applied force is
increased, the value of (hCGo)T .decreases. This
would suggest that, to improve controllability of
a spacesuit on the surface of a very low-gravity
body, the center of gravity should be positioned
as close to the front of the chest as possible, thus
possibly allowing the crewmember to use his or
her arms and legs to more effectively control the
translational and rotational motions.
Knowing that it is possible to push vertically
with enough force to cause unimpeded rotational
motion, it is worth comparing the full motion of
1) the impeded rotational-motion, and 2) the
unimpeded rotational-motion trajectories.
10.9 Translational/Rotational Motion: A Full
Trajectory Analysis
The full trajectory motion for all three
applied forces can be completed, but for
illustration purposes, this paper will focus on the
trajectory associated with F = 22.2 N. To show
two cases, and the resulting motion on the
astronaut, this section will begin with the
impeded trajectory of hCGo = 0.35 meters, and
then examine that of hCGo = (hCGo)T = 0.416
meters. In this way, one can see the effect that
foot-contact with the surface has on the
crewmember’s trajectory.
Additionally, due to the fact that no collision
is elastic, it is advisable to assume some level of
rotational damping. Damping will be caused by
such variables as the flexibility of the
crewmember, the density and composition of the
very low-gravity body’s surface, and the
pliability of the segments of the spacesuit that
contact the surface.
However, since it is
dependent on so many variables, the first two
trajectory analyses (as stated above) will
arbitrarily assume that rotational damping will be
50 percent; in other words, each time the EMU
contacts the surface, it loses 50 percent of its
angular momentum.
Yet it is important to show that any
arbitrarily chosen value for rotational damping
has a considerable impact on the trajectory. The
third trajectory analysis in this section will return
to a value of hCGo = 0.35 meters, but will
consider any collision between the astronaut and
the surface losing only 25% of it angular
momentum. While this again simply chooses
another arbitrary value for rotational damping, it
best demonstrates the significant influence that
varied levels of damping have on the rotational
motion.
10.10 Trajectory Analysis: hCGo = 0.35 meters
The parameters resulting from the first time
the astronaut contacts the surface were calculated
in Section 10.6 and reported in Table 12. The
rotation is traveling counter-clockwise, but upon
contact with the surface (at t = 4.1 seconds), it
changes direction to clockwise, retaining onehalf of its angular momentum. This, along with
the rest of the flight trajectory, is shown
graphically in Figure 9. Note that the horizontal
arrangement does not indicate horizontal motion;
the translational motion is confined to the +/- h
direction (normal to the surface).
+h
+ω
-θ
hCG
Surface of Phobos
t (sec)
t=2.1
t=4.1
t=18.0
t=31.94
t=45.9
t=60.7
t=63.2
Figure 9. Illustration of the flight trajectory, F=22.2N, hCGo=0.35 m
t (sec)
hCG (m)
v
(m/s)
θ (rad)
ω (rad/s)
2.1
0.513
0.155
-0.245
-0.233
4.1
0.809
0.134
-0.703
0.117
18.0
2.326
0.061
0.924
0.117
31.94
(hCG)Max = 2.832
0.0
2.549
0.117
45.9
2.329
-0.084
4.107
0.117
60.7
0.679
-0.161
5.901
0.117
63.2
0.289
?
6.192
?
Table 13. Flight trajectory parameters, F=22.2N, hCGo=0.35 m
Table 13 presents the various parameters of
motion for a number of points throughout the
trajectory. At t = 2.1 seconds, the crewmember
has just completed the push-off and enters the
free-flight portion of the trajectory.
Figure 9 illustrates the considerable risk to
the crewmember due to the rotation about the
CG; at times, the astronaut cannot actually see
the surface. Additionally, at t = 60.7 seconds,
the CG is at a height of 0.679 meters – roughly
the distance from the CG to the palm of a fully
extended arm – and has rotated nearly 340
degrees. In this orientation, the astronaut’s
hands will be the first body part to touch the
surface. Depending on the amount of energy the
astronaut can absorb with his or her arms, it is
likely that the collision with the surface will
cause the astronaut to oscillate upward again,
repeating this motion until all energy has been
dissipated.
10.11 Trajectory Analysis: hCGo = 0.416 meters
To understand how the value of hCGo affects
the trajectory’s rotation, it is necessary to
examine a case where hCGo ≥ (hCGo)T.. Per
Section 10.8, where hCGo = (hCGo)T = 0.416 m,
the astronaut’s feet will come within one
millimeter of the surface but will not make
contact during the positive vertical translation.
This will significantly alter the trajectory, as the
rotation will continue in a counter-clockwise
direction until the astronaut has come back down
and some portion of the spacesuit makes initial
contact with the surface. Figure 10 depicts this
motion.
+h
-θ
+ω
hCG
Surface of Phobos
t (sec)
t=2.1
t=5.74
t=18.0
t=31.94
t=45.9
t=58.9
t=
Figure 10. Illustration of the flight trajectory, F=22.2N, hCGo=0.416 m
t (sec)
hCG (m)
v
(m/s)
θ (rad)
ω (rad/s)
2.1
0.583
0.155
-0.245
-0.233
5.74
1.105
0.125
-1.084
-0.233
18.0
2.392
0.061
-3.951
-0.233
31.94
hCGmax= 2.898
0.0
-7.200
-0.233
45.9
2.391
-0.084
-10.454
-0.233
58.9
1.001
-0.152
-13.494
0.117
57.8
0.28
?
-12.791
?
Table 14. Flight trajectory parameters, F=22.2N, hCGo=0.416 m
Due to the fact that this rotation is
unimpeded, the astronaut continues to rotate
counter-clockwise throughout the positive and
negative vertical translations, until the feet
contact the ground at t = 58.9 seconds. At this
point, the astronaut has rotated through nearly
775 degrees of counter-clockwise rotation over
almost one minute of free flight.
In the
descending phase of this trajectory, the initial
contact with the surface results in continued
clockwise rotation.
However, if the astronaut’s feet do not skid
across the surface, the rotation about the CG
shifts to rotation about the feet, since the rate of
negative vertical translational motion is greater
than the rotation rate. The feet thus remain in
contact with the surface, and the relative motion
introduces some small amount of horizontal
motion of the CG to the right. Again, depending
on the astronaut’s ability to damped that motion
as the hands come into contact with the surface,
the clockwise rotation may shift back about the
CG, causing the astronaut’s legs to cartwheel
overhead.
Choosing any value of hCGo greater than
(hCGo)T has less effect on the trajectory. For a
value of hCGo = (hCGo)Max = 0.527 m, the
astronaut’s feet first contact the surface at t =
59.3 seconds (vice 58.9 s), θ = -13.578 rad, and
hCG = 1.061 m. The one time this may be a more
significant issue is if initial contact with the
surface occurs very near to θ = ±π/2 rad, where a
slight difference in contact angle results in the
difference between the crewmember ending up
on his or her back rather than facing the surface.
The higher starting point results in the astronaut
striking the surface in a slightly more upright
orientation, but results in a very similar
clockwise rotation about the feet toward the
surface.
10.12 Trajectory Analysis: Altered Damping
For the third and final trajectory analysis,
the value of the initial CG height will return to
hCGo = 0.35 m, as it was in Section 10.10.
However, the rotational damping will be reduced
from an arbitrarily chosen value of 50% to
another arbitrarily chosen value of 25%. The
initial motion will be identical to that of Section
10.10. The difference will arise after time t = 4.1
seconds, when the counter-clockwise rotation
alters to clockwise rotation resulting from the
collision between the astronaut’s feet and the
surface. Rather than maintain one-half of its
angular momentum, however, the altered value
for rotational damping allows the rotation to
maintain three-quarters of its momentum,
significantly changing the astronaut’s trajectory.
Figure 11 below depicts the altered damping
trajectory.
+h
+ω
-θ
hCG
Surface of Phobos
t (sec)
t=2.1
t=4.1
t=18.0
t=31.94
t=45.9
t=63.3
Figure 11. Illustration of the flight trajectory, F=22.2N, hCGo=0.35 m, 25% rotational damping
t (sec)
hCG (m)
v
(m/s)
θ (rad)
ω (rad/s)
2.1
0.513
0.155
-0.245
-0.233
4.1
0.809
0.134
-0.703
0.175
18.0
2.326
0.061
1.736
0.175
31.94
(hCG)Max = 2.832
0.0
4.173
0.175
45.9
2.329
-0.084
6.605
0.117
63.3
0.279
?
9.649
?
Table 15. Flight trajectory parameters, F=22.2N, hCGo=0.35 m, 25% rotational damping
With less rotational damping, the astronaut
now rotates through over 550 degrees in the
clockwise direction, nearly 200 degrees more
rotation than with the rotational damping at 50%.
In this third case, when the astronaut makes
contact with the surface during the negative
vertical translation, the astronaut is on his or her
back, and when the astronaut’s heels strike the
surface, it initiates another counter-clockwise
rotation, placing the astronaut farther onto his or
her back. With no way to damp the vertical
motion, one would expect the astronaut to strike
the surface and oscillate upward.
Additionally, there is no way to predict
whether the astronaut’s heels will skid across the
surface, and thus without knowing the variables
that effect damping, it is pointless to speculate
on the values of
and ω after the heels have
made contact. From the illustration, the only
necessary information can be easily deduced; an
astronaut landing on his or her back in an
environment with rough-hewn features faces a
great safety risk.
10.13 Trajectory Analysis: Conclusions
More than either of the other illustrations,
this third case shows the dangers of not
maintaining attitude control. Altering the input
force, its duration, or its angle relative to the
surface changes the trajectory. Recall, as well,
that Phobos was modeled as a sphere with
constant bulk density, but that will not be the
reality for any low-gravity body. Odd shapes,
voids throughout the substructure, and varying
bulk densities due to composition will create
inconsistent gravitational fields. Additionally,
changing latitude away from the equator reduces
centripetal acceleration, thus increasing the
gravitation sensed by the astronaut.
Move away from Phobos, to a smaller body,
and the motion becomes more problematic. All
other parameters equal, if a potential target had a
mean radius just one-quarter of that of Phobos
(i.e., r = 2.275 km; still large by NEO standards),
the total gravitation sensed by the astronaut
would reduce to gTOT = 6.16 x 10-4 m/s2. Using
the calculation method previously described, and
taking F = 22.2 N, (hCG)Max would change from a
height of about 2.8 meters to more than 20
meters high, taking over 250 seconds to reach
that height. This means that an astronaut
applying that moderate force would be in freeflight for over eight minutes.
To further demonstrate the vast differences
caused from the numerous variable parameters,
double the period of rotation of this new body.
The altered period of rotation changes the
centripetal acceleration at the surface. The
period of rotation now becomes 15.31 hours, the
value for the sensed gravitation increases to gTOT
= 1.163 x 10-3 m/s2, (hCG)Max = 10.89 meters, and
the time to the maximum height is 135 seconds.
While this is not much of an improvement
for the astronaut’s working conditions, it does
demonstrate the effect that so many variable
parameters have on surface mobility. The
challenge, then, for a very low-gravity
exploration architecture, is to develop hardware
and operational concepts that allow widely
varying potential targets to remain within the
program’s capabilities.
11. ISS Utilization for Surface Mobility
Research
Hardware
and
operational-concept
development for a low-gravity exploration
program would be well served to employ a
progressive approach, utilizing all available
resources to test out the devices and techniques
that will be needed to both keep astronauts safe
and allow them to be productive. Sometimes
those two objectives – safety and productivity –
run counter to each other. In an effort to make
astronauts safer, for example, alterations to a
spacesuit design may reduce dexterity, visibility,
even operability, leading to not only a less
efficient working environment, but sometimes
even inducing additional risk. Due to the nature
of the exploration destinations, and even with
robotic precursor missions paving the way to a
particular target, unknowns are inevitable. The
only way to overcome those unknown unknowns
is by developing and vetting an array of both
hardware and concepts, thus providing the
astronauts on location the opportunity to choose
the tools and techniques best suited to the local
environment and the exploration objectives.
NASA has a number of assets for
development and testing, including the Neutral
Buoyancy Laboratory and the Virtual-Reality
Laboratory, both of which are housed at NASA’s
Johnson Space Center. Each facility, however,
will have benefits and deficiencies with respect
to very low-gravity exploration development.
The zero-gravity aircraft that NASA utilizes to
test and validate concepts and hardware, for
example, are capable of flying parabolic flight
paths that can imitate roughly any gravitational
force, from microgravity to lunar gravitation and
beyond. The parabolas, however, only yield
continuous test-time on the order of 30 seconds.
Additionally, since the inside of an aircraft is a
confined environment, it is not an ideal setting to
test large scale, long-duration motion response
hardware.
NASA understood these limitations when it
addressed the design and validation of the
SAFER. Built as a self-rescue device for an
astronaut that becomes inadvertently separated
from the spacecraft during an EVA, SAFER uses
cold-gas nitrogen thrusters to provide attitude
control and propulsion. Instead of attempting to
validate its design in a simulated environment,
NASA chose to test it in low-Earth orbit as part
of a Designated Test Objective (DTO). Flying
on STS-64 in 1994, STS-88 in 1998 and STS-92
in 2000 [8], the SAFER was used during an EVA
on each flight, to improve the design and validate
it as a redundant safety system. In a similar
manner, very low-gravity EVA hardware and
operational concepts can and should be tested
through a series of EVAs conducted onboard the
International Space Station.
11.1 Questions Requiring Resolution
The theoretical analysis of motion presented
in this paper is just that – theoretical. As such, a
number of assumptions were necessarily made to
complete the analysis. However, it is important
to determine actual motions, to identify the
perceived hardware and operational-concept
needs from actual needs. Time and again, the
men and women of NASA’s astronaut corps
have demonstrated an acute adaptability to
working in the EVA environment; some of the
perceived challenges regarding very low-gravity
EVA may be less significant if astronauts can
minimize the severity of those challenges simply
through adaptation.
A series of tests performed through the use
of ISS-based EVAs could provide information
regarding the capabilities of the human-in-theloop, along with valuable feedback that focuses
development of hardware and concepts to meet
the actual low-gravity EVA needs, rather than
theoretically-perceived needs.
The testing
should be progressive, as well, and the program
be given sufficient time to use the information
learned from each test to develop and design the
objectives and hardware for subsequent testing.
In this way, the ISS becomes a test-bed for very
low-gravity EVA research.
First, however, some of the basic questions
need to be answered, to help define such a lowEarth orbit research program. This paper has
focused primarily on surface mobility, because it
is the quintessential core competency for very
low-gravity body exploration. If an astronaut
cannot safely and efficiently move about on the
surface of a very low-gravity body, a program
built on the exploration of such bodies is futile.
Geological exploration is most important when
the samples are collected within the geological
context of the body. This means that, ideally,
samples are taken from various, distinct
locations about the body.
Therefore an
operational concept that restricts astronauts to
one small area – working from a platform
attached to a spacecraft that has landed on the
surface, for example – is less than ideal. Every
effort should be made to provide the astronauts
the greatest degree of freedom possible. The
answer to each question will likely raise more
questions, but by giving this program sufficient
time to mature, the evolution will be natural, and
the likelihood of success for the first very lowgravity EVA will increase exponentially.
The following are but some of the questions
regarding surface mobility that may be answered
through an ISS-based EVA research program.
The list is in no way comprehensive, but it does
attempt to capture the concerns addressed in the
previous pages of this paper regarding the risks
to the astronaut as he or she attempts to navigate
one’s way over the surface of a very low-gravity
body.
1) What is the minimum level of gravitation
that an astronaut can safely and effectively
operate in without any sort of restraint
system (no tethers, no handholds, no anchors
of any kind)?
2) How well and how quickly can an astronaut
adapt to using only the minimum amount of
force needed to maneuver about on a lowgravity body?
3) How well and how quickly can an astronaut
learn to direct the applied forces to limit
vertical linear motion without the aid of an
attitude control system (i.e., moving
horizontally over the surface rather than up
and down)?
4) Using one’s hands to propel the spacesuit
across the surface places the gloves at risk of
a cut or tear that could lead to the
termination of an EVA. What design
alterations would negate this risk without
significantly degrading the dexterity needed
to meet exploration objectives (e.g.,
removable gauntlets, reinforced glove
substructure, etc.)?
5) How well can an astronaut absorb and
dissipate the energy encountered during the
negative vertical translational motion, to
avoid bouncing off the surface?
6) What is the optimal way to design a
spacesuit and the ancillary hardware that
allows an astronaut to remain in the prone
position without the majority of spacesuit
touching the surface (e.g., bi-pods on the
feet, braces extending from the spacesuit
chest, etc.)?
7) How does a damper system integrate with
the rest of the spacesuit? While an astronaut
may be able to control negative vertical
translational motion with one’s arms, is it
wise to have pressurized gloves in contact
with the surface as the entire weight of the
spacesuit is descending toward the surface?
8) How effectively can an astronaut use one’s
legs to create a rotational moment that
opposes the rotational moment caused by the
arms during push-off?
9) Can a redesign of the spacesuit boots assist
further in controlling rotation and protect the
pressurized volume from cuts and tears (e.g.,
flexible tips that act as springs, etc.)?
10) If an astronaut can use one’s legs to help
control rotation, is there an optimal θo (angle
of the suit relative to the surface; for the
previous analyses, θo = 0 radians) that best
allows the astronaut to control rotation?
11) An attitude control system of some kind will
be necessary for any operational concept
that gives the astronaut six-degree freedom
of movement. What are the minimum
required capabilities of such a system (e.g.,
does it control only rotation, does it only
control pitch and roll but not yaw, does it
use cold gas jets which may plume dustcovered surfaces, etc.)?
STS-98, in 2001 [21]. Figure 13 shows a flyaround photo taken during STS-127, in 2009
[18]. In the region on the Lab noted as the
primary area of interest in Figure 11, two rows
of handrails run aft from the forward end-cone.
12) In the prone position, the visor may come
within very close proximity of the surface.
What helmet/visor design alterations are
necessary to improve safety and at the same
time limit the loss of visibility?
13) In the prone position, it will be difficult for
the astronaut to see forward or to the sides.
What sort of onboard visualization system
will aid in mobility, spatial awareness and
photo documentation (e.g., camera views
integrated into a heads-up display visible on
the inside of the visor, a helmet with 360
degrees of visibility, etc.)?
These are the types of questions that will
need to be addressed before a spacesuit that is
optimally designed for very low-gravity
exploration can be built. From the previous
analyses, the motion of an astronaut on the
surface of Phobos has the potential to be
hazardous. Only by examining the capabilities
of the human in the loop with respect to force
inputs, and the level of ability to control the
motion, can spacesuit design parameters be
defined.
Without first obtaining this
experimental and empirical data, the spacesuit
design will force the operational concepts to find
ways to work around the deficiencies of the
spacesuit.
The following is one very simplistic way to
at least begin to determine the answers to some
of the questions above. It may not be the ideal
experimental setup, but it is presented here to
show one way of using ISS as an EVA research
facility.
11.2 A Simplistic EVA Evaluation Aboard ISS
The zenith side of the U.S. Destiny
Laboratory (Lab) may be a feasible location to
perform a series of Designated Test Objectives.
The Lab itself is about 8.5 meters long, with a
diameter of about 4.4 meters [8]. Figure 12
shows the Lab as it was being installed during
Figure 12. ISS Destiny Laboratory during install, STS-98[21]
Port
Fwd
Primary Area
of Interest
Figure 13. ISS Destiny Laboratory, STS-127 fly-around[18]
Knowing the basic dimensions of the
module, it can be estimated from Figures 12 and
13 that the handrail rows are about 1.3 meters
apart.
Additionally, the distance from the
forward end-cone to the point where the S0 truss
segment attaches to the Lab is about 4.85 meters.
In this region between the handrail rows, an
astronaut could use a system of slide wires and
cables to evaluate many of the topics discussed
in Section 11.1.
To begin, an astronaut would install a slide
wire stanchion onto the end of each handrail row,
running forward and aft. This would allow two
separate, parallel slide wires to run from the
forward end cone to a handrail stanchion near the
location where the S0 truss segment attaches to
the Lab, an overall length of approximately 4.85
meters. Then, an astronaut would employ a
modified work station that attaches to the front
of the EMU. This work station has two
armatures, each with a reel housing at the end of
it. The reel housings are in line with the CG
with respect to the z-axis of the EMU. Within
each reel housing is an inelastic cable connected
to a constant-force spring. The astronaut
positions him or herself between the slide wires,
and attaches each inelastic cable to the respective
slide wire by a small hook at the end of the
inelastic cable. Figure 14 shows the astronaut
between the slide wires, with the inelastic cables
attached. The astronaut’s hands are resting on
the top of the Lab as he waits for the system to
equilibrate.
Zenith
Port
Modified
Work Station
Reel
Housing
Slide
Wire
Inelastic
Cable
Figure 14. Astronaut positioned on Lab zenith
By knowing the relative geometry, it is
possible to equip each reel with the appropriate
constant-force spring to mimic any desired
gravitation. The geometric dimensions in Figure
15 are rough estimations made only for
illustration purposes. As is evident, if any such
DTO were to be performed on the ISS, a great
deal more thought and consideration would go
into every aspect of the experiment. This simple
example, for instance, does not account for the
variations in slide wire tension, and assumes
cables and slide wires that are perfectly inelastic.
The required value of Fv is one-half the force of
gravity on, for this example, Phobos.
=
= 0.781
This is the amount of force required of each
constant-force spring to mimic the gravitation
that an astronaut would experience on Phobos.
As the astronaut pushes off from the Lab, the
reel housings are mounted on swivels, so that his
or her rotation is not impeded by the cabling
holding the modified work station to the slide
wire. By employing the slide wire, an astronaut
can translate both vertically and horizontally
over the entire slide wire length. In this way, an
astronaut can evaluate and offer feedback
regarding many of the questions listed above.
To further improve results, instrumentation to
measure linear and rotational rates, and the input
forces that generate those rates would be needed.
Figure 16 shows an astronaut after he has
pushed off from the Lab. It is quickly apparent
that even an evaluation like this has limitations.
As the astronaut rotates, the location of the reel
housings mounted to the modified work station
moves farther out of alignment with the CG.
Designing a modified work station that places
the heel housings as close to the CG as possible
along the z-axis will help alleviate this problem.
Yet even in the simple configuration presented in
Figure 16, an astronaut will be able to provide
valuable commentary regarding his or her ability
to use one’s legs to dampen rotational motion,
the ability to limit input forces, and whether the
astronaut can direct those forces at will.
Zenith
Fwd
Zenith
Modified
Work Station
Port
Reel
Housing
hCG
Slide
Wire
Fv
Fv
Inelastic
Cable
1.3 m
U.S. Laboratory
U.S. Laboratory
Figure 15. Experiment geometry
(Astronaut not shown for clarity)
(18)
Figure 16. Free-floating astronaut under
simulated Phobos gravitation
Additionally, altering the force of the
constant-force spring system will provide
astronauts an opportunity to weigh in, through
crew consensus, on what is the minimum
acceptable gravitational force that would not
require a restraint system. This alone will be a
significantly valuable piece of information, as it
may rule out a large number of potential targets.
The feedback provided by the astronauts
will help bound some of the challenges.
Understanding the adaptability of an astronaut in
such an environment – even when simulated –
and quantifying the level of control said
astronaut can attain will have significant impacts
on system requirements. By understanding the
level of control, designers do not need to
assumptions when designing accompanying
hardware such as motion damping systems and
attitude control systems.
The greater the
astronaut’s capabilities to maintain body control,
and the greater the understanding of those
abilities, the more precisely designed those
accompanying automated systems can be. And
any time a system can be designed to actual
needs, rather than assumptions, results in a more
efficient system.
As testing progresses, this apparatus can be
used to test various damping systems, attitude
control systems, and eventually spacesuits
designed for very low-gravity EVA. It is also
very likely that creative engineers and scientists
can devise other ways to configure worksites
aboard ISS to cater to this type of testing. It
should be noted that EVA time is valuable
aboard ISS, and these evaluations, at least in the
beginning, are likely to be added to an EVA
timeline that is primarily focused on repairing
ISS hardware. As such, it is imperative that the
configuration, setup and execution of these
Designated Test Objectives is simple and can be
accomplished as quickly as possible. An idyllic
configuration would be one that utilizes an area
of ISS seldom traveled, where the experimental
apparatus can remain deployed between EVAs.
This gives the astronauts the opportunity to meet
more research objectives as time allows during
EVAs primarily focused on other, unrelated
tasks. In this way, ISS can be that unparalleled
research facility that paves the way for the future
exploration of the solar system.
12. Spacesuit Orientation Methodology
As was mentioned earlier, there is still the
open question of how to secure the spacesuit in a
stable prone position, thus permitting the above
described operational concept for surface
mobility. The surface of a NEO presents a
plethora of dangers to a spacewalking astronaut,
not the least of which is sharp, jagged rocks.
Contact with the surface must therefore be
minimized. This paper will present one possible
way to overcome this significant issue.
Figures 17 and 18 represent two views of the
same crewmember.
In these figures, the
spacesuit is equipped with a system the author
has termed the Cricket System. Four legs extend
from the primary life support system (i.e., the
backpack), designed to a prescribed length based
on both the results of ISS evaluations regarding
adaptability, and crewmember stature. These
legs act, in essence, like the legs of a table,
supporting the spacesuit in an orientation that
optimizes both surface translation and geological
exploration. Even at very low-gravity, the
crewmember will descend to the surface over
time. The Cricket System eliminates that falling
motion, stabilizing the spacesuit in a relaxed
orientation, allowing the astronaut to focus less
on body control and more on geological
exploration.
+h
+ω
NEO Surface
Figure 17. Cricket System: side view
The Cricket System also possesses a number
of other desirable attributes. The biggest danger
of this system is that a crewmember propels
himself or herself across the surface of a very
low-gravity body, and in the process, rotates to a
point where he or she ends up on one’s back, like
a tortoise. The feet of this system, however,
could be geometrically designed to increase
stability and impart a righting moment, should
the crewmember land askew.
The legs of the Cricket System also become
an excellent platform, if designed for operability,
for the tools necessary for geological
exploration. Deployed sample collection bags
would ease that process for an astronaut, as
would tool stowage, thus keeping the most
essential tools within easy reach of a prospecting
crewmember.
wearing a modified boot. The tip of the boot
would be flexible, much like the fins worn by a
scuba diver.
+ω
NEO Surface
Figure 18. Cricket System: boot tip modification
Figure 17. Cricket System: top view
(crewmember facing down toward the surface)
As a passive system, it has desirable
attributes. However, if testing showed that a
crewmember would need assistance in damping
vertical motion, the Cricket System possesses
the potential to support such a system. By
incorporating a damping system into the feet, the
Cricket System could absorb much of the
negative vertical motion of a translating
crewmember, thus reducing the amount of force
the crewmember would need to absorb with his
or her hands. In this way, the crewmember
would, in theory, only be touching the surface to
collect geological samples, and to begin a
translation.
Regarding rotations during translation,
designers may be able to incorporate the
damping system into an overall, active attitude
control system, using some of the stored energy
from each “landing” to impart a force opposite
the sensed rotation caused by a crewmember’s
arms at the beginning of the next translation. In
this way, the spacesuit itself may require little or
no other attitude control.
Another option for rotational control, in
conjunction with the Cricket System, would be to
find a way to incorporate the capabilities of the
astronaut in further influencing rotational
motion.
Figure 18 depicts a crewmember
The concept would be to utilize the spring
force of the boot tips, to counteract the rotational
motion induced by a crewmember pushing off
the surface of a very low-gravity body with his
or her hands. The astronaut would employ his or
her legs to generate a small force that would, in
theory, create and equal and opposite torque
upon the spacesuit; the negative torque generated
by the arms during push off would be nullified
by the positive torque generated by the legs. In
this way, a crewmember would need no active
attitude control system; he or she would simply
bound, face down, across the surface of a NEO.
This is obviously a design that would need
to be tested in either a simulated zero-g
environment – such as aboard a zero-g aircraft –
or tested as part of a DTO aboard the ISS. The
necessity for a DTO would be not only to
determine the optimal flexibility of the boot tips,
but to determine the adaptability of the
astronauts to utilize this system in a controlled
manner.
One immediate and obvious challenge to the
Cricket System, however, would be its method of
deployment, and the associated complexity. In
the configuration shown in the figures, a
crewmember could not fit through a nominally
designed airlock hatch. Thus the Cricket System
would need to have the ability to move from a
stowed position to a deployed position, thus
allowing an astronaut to move through the
airlock hatch without worrying about damaging
hatch seals. It may be as simple as designing
each leg to rotate about its connection point, to
streamline the system with the crewmember, or it
may be as complicated as some sort of telescopic
system. This is obviously open work, and a
large number of questions would need to be
answered to decide on the best methodology, but
nonetheless it is not insurmountable by any
means.
There is certainly a vast amount of open
work to solve the issues associated with surface
mobility. How it should be done, and even
whether it actually can be done, loom large for
any exploration architecture that counts very
low-gravity bodies as possible destinations. By
synergizing the creative engineering talents in
the aerospace industry with the operational
experience gained through Apollo, the space
shuttle and the ISS programs, however, there is
no doubt that methods can be devised that will at
least allow humanity to attempt the exploration
of NEOs and other small celestial bodies. The
question then becomes “what sorts of activities
are the explorers of these bodies going to
conduct?” The starting point for that answer
again comes back to the Apollo program.
13. Apollo Scientific Experimentation Setup
The first EVA of every Apollo lunar surface
mission had the highest priority tasks assigned to
it, in case the crew was forced from the surface
before the nominal end of the mission. Some of
these tasks – such as Lunar Flag Placement and
TV camera setup – were not scientific in nature,
but human in nature. Others, however, were
purely scientific. One such activity was the
Contingency Sample Collection.
Beginning right away with Apollo 11, and
conducted on every mission through Apollo 15,
the crew, upon reaching the surface, would
collect surface rock and soil samples. Only a
few minutes would be spent on this task, but the
notion was that, should the crew need to abort
the EVA and leave the surface of the Moon,
there would be some geological samples aboard
the LM for return to Earth. Apollo 16 and 17 did
not perform this task; the author conjectures that
the engineering team was confident that the crew
would not be forced to leave the surface on such
short notice, and thus chose to use the valuable
EVA time for other tasks.
One of these other such tasks that was
conducted on every Apollo mission, was the
setup of the surface experimentation package.
Apollo 11 deployed the Early Apollo Scientific
Experiments Package (EASEP), consisting of a
laser ranging retroflector, passive seismic
experiment and a lunar dust detector. The
experiments were easy to deploy but the crew
found it difficult to find a level spot near the LM
for deployment. Apollo 12 through 17 each
deployed
an
Apollo
Lunar
Surface
Experimentation Package (ALSEP). The exact
experiments that comprised the package evolved
from flight to flight; some experiments were
often repeated, while others were unique to a
specific mission. Apollo 12 had six experiments,
including a solar wind spectrometer and a lunar
surface magnetometer. Apollo 15 had eight
experiments, including the solar wind
spectrometer, as well as a cold cathode gauge
and a superthermal ion detector. Apollo 17 also
had only six experiments, but its package
consisted of such experiments as a lunar surface
gravimeter and a lunar ejecta and meteorites
experiment.
Difficulties in deploying the ALSEP were
often encountered, and the crew was forced to
spend valuable EVA time troubleshooting the
contingencies. Apollo 14 had similar difficulties
to Apollo 11 regarding finding a level spot for
deployment, while on Apollo 15, the crew had
difficulty drilling holes into the lunar surface for
a heat flow experiment.
For NEO exploration, the number of
experiments that will require deployment by
spacewalking astronauts is dependent on the
capabilities of the robotic precursor missions.
Should a robotic mission be capable of
performing
spectroscopy,
or
measuring
gravitational fields, experiments of this nature
may be redundant, making space available for
unique research on another aspect of the NEO.
For example, Abell et. al. suggests that the
“active detonation of a kinetic energy experiment
after deployment of a seismic network
would…serve to measure the interior of the NEO
while gaining insights into the effects of crater
excavation” [2]. The deployment of such a
network, for example, may require the real-time
assessment that can only come from the feedback
the geological team on Earth gets from the
astronauts in situ.
In this way, it is difficult to predict the types
of experiments that the science teams will want
to conduct, but it is easy to imagine that some
portion of the EVA time that the astronauts
spend on the surface of a NEO will be dedicated
to the deployment of scientific experiments. Due
to the challenges associated with working in an
environment much less stable than the surface of
the Moon, however, experiment designers should
pay particular attention to ensuring simple
deployment, to minimize both crew time that
would be lost to troubleshooting a contingency,
and the possibility that the contingency cannot
be overcome and the deployment of the
experiment is aborted.
14. Field Geology
The mantra of EVA aboard the ISS is that
slow is fast. In the micro-gravity environment of
low-Earth orbit, a spacewalking crewmember
finds it easy to start his or her motion, and
difficult to stop that motion. Utilizing handrails
to traverse about the exterior of the ISS,
crewmembers rely on upper body strength and
leverage to arrest their motion. Stopping when
both hands are very close together is much
harder than when the hands are separated.
Stopping becomes even easier when the hands
are also out of plane, providing leverage. And as
is obvious, any input rate to begin a motion must
be nullified to stop a motion. Therefore, the
mantra that slow is fast.
By beginning every motion slowly, the rate
is low. A slow rate of motion is easier to stop,
requiring less muscle input. This may seems
insignificant at first glance, as the environment is
weightless and an astronaut must simply
maintain control of the overall mass of him or
herself plus the spacesuit. However, over six
hours, fatigue plays a major role in the level of
success attained by the spacewalk. Tired hands
struggle to actuate tools, and fatigue in general
often leads to a loss of focus. A six and a half
hour EVA that has been conducted flawlessly for
first 95% of the spacewalk can turn in an instant,
when fatigue results in carelessness and a critical
piece of hardware is accidentally set adrift. Not
only is the item lost overboard, but it becomes a
recontact danger for the ISS itself, sometimes
requiring the station to perform a costly (and
unplanned) avoidance maneuver. These induced
risks to crew and vehicle are minimized through
extensive training, and by instilling the belief in
all spacewalking crewmembers that conserving
energy is essential to success, and that the most
effective way to conserve energy is by
remembering that slow is fast.
This same philosophy will be invaluable on
the surface of a very low-gravity body. Whether
moving across the surface or seeking the perfect
position to photograph a geological find, moving
slowly conserves energy and allows an astronaut
to maintain a higher level of body control. For
an astronaut exploring the surface of a NEO,
however, this mantra will likely be most
significant with respect to field geology. Every
movement across the surface will be, in essence,
to reposition oneself to continue the geological
survey and sample collection. This paper will
attempt to identify the major types of scientific
activities conducted on the lunar surface by the
Apollo astronauts, which represent an excellent
starting point for NEO surface-exploration
operational concepts.
However, whether a
crewmember is collecting surface samples,
attempting to dig a small trench, or taking core
samples, the success of field geology will depend
entirely on a crewmember’s ability to maintain
good body control. Moving slowly will allow an
astronaut to work efficiently and effectively, thus
likely resulting in a high quantity of quality field
research.
14.1 Surface Sample Collection
This field geology activity was the simplest
for the Apollo astronauts, and will be the
simplest for the astronauts exploring the surface
of a NEO. Every Apollo mission performed this
task, which can be broken into two categories:
rock samples and soil samples.
Using the training received in geology prior
to flight, the Apollo astronauts attempted to
collect unique rock samples from the various
locations visited. Using such tools as the tongs,
the rake and the scoop, as well as their hands,
rocks were collected from nearly every location
visited; and when time permitted, the crews often
worked diligently to document the sample
collection from each site through words and
photography, to help maintain the geological
context. To understand the quantity of samples
collected, consider EVA2 from Apollo 15.
During a geological traverse, the crew stopped at
five separate stations. At the third station alone,
the crew collected 93 samples, including the
famous Genesis Rock.
Similarly, the crews used sample collection
bags to stow soil samples, most often scraped up
from the surface using the scoop. The length of
the tools, in conjunction with the limited
flexibility of the Apollo spacesuit, sometimes
produced inefficient results. At times the crews
struggled pouring contents into sample return
bags, leading to more exertion (and frustration)
than necessary. On Apollo 16, the crew reported
that the straps holding the sample bags to the
spacesuit PLSS would not stay tight, and
considerable time throughout the day was spent
cinching these straps back in place.
Additionally, this same crew reported during
EVA3 that the sample bags were top-heavy and
when they were loaded, had a tendency to tip
over, thus spilling some of the sample back to
the surface.
Many of these challenges were the direct
result of the inflexibility of the spacesuit, and
therefore the need to use digging-type and
grabbing-type tools. This further signifies the
importance of developing some method of
positioning the crewmembers nearer to the
surface, where they can use their hands in
concert with short-handled (and thus dexterous)
tools. By designing tool stowage and sample
collection stowage into the legs of the Cricket
System, for example, an astronaut would be able
to expend his or her energy on the most
important tasks at hand, namely the efficient
collection and documentation of surface samples,
rather than searching for operational workarounds to hardware limitations.
14.2 Trench Sample Collection
To better understand the lunar composition,
scientists determined the need to take soil
samples from areas below the surface of the
Moon. Beginning on EVA2 of Apollo 12,
astronauts began taking soil samples from the
bottom of shallow trenches, which they dug
using the long-handled scoop. The method for
sample collection mirrored that for the surface
soil samples, except for the fact that the astronaut
first had to dig a shallow trench, and then take
the soil sample from the bottom of that trench.
During that first attempt to dig a trench, on
Apollo 12, the crew reported that they were able
to dig down 20 centimeters, and that depth was
only limited by the tool’s handle length; had they
longer tools, the crew reported that they could
have dug deeper. Conversely, during Apollo 15,
Irwin reported that he had only been able to dig a
trench to a depth of about one foot; after that
point, he stated that it felt as though he was
scraping bedrock.
For a NEO astronaut, this task would not be
so simple. If the surface density was low, and
the astronaut moved slowly, it may be possible
to dig a trench, thus allowing soil samples to be
taken below the surface. However, without some
sort of restraint system in place, any force the
astronaut imparts while pushing a trenching tool
against the surface will likely cause him or her to
move. As an astronaut traverses about on a
NEO, this could result in inconsistent sample
collection, as varying surface compositions may
make trenching impossible for an unrestrained
crewmember.
During the development phase, testing could
be conducted to bound the abilities of an
astronaut to trench the soil as a function of the
effective gravitational force. Such testing would
provide hardware designers with the necessary
feedback
to
determine
optimal
tool
configurations, as well as whether trenching
needs to be an action decoupled from the
astronaut.
If an astronaut in a very low-gravity
environment cannot dig down into the soil
without pushing himself or herself away from
the worksite, it may be necessary to develop
tools that can be placed on the surface and dig a
trench autonomously. This introduces another
level of complexity into the hardware design,
however. With all the various types of NEOs –
some with a rocky composition, some merely
rubble piles [11], some comprised primarily of
metals – how would such an autonomous tool
attach itself to the surface in a manner sufficient
to react the loads encountered while trenching
the soil.
This question is much larger than just
trenching tools, however. Taking core samples,
for instance, and drilling – discussed in the
following sections – likely need to be decoupled
from the astronaut, and thus also need
attachment methods to the various types of
NEOs. Even more significantly, though, is
whether an attachment system can be devised
that allows the visiting vehicle to maintain
contact with the surface, rather than flying in
some station-keeping position. This is a question
that will likely generate a considerable amount of
research in the coming years.
14.3 Core Sample Collection
Of the often-performed geological tasks
from the Apollo program, this is likely the most
difficult to transpose directly to a NEO. Every
Apollo mission extracted core samples, starting
with Apollo 11. And to the credit of the
hardware designers, only Apollo 11 had
significant issues with core sample collection.
The Apollo method for core sample
collection was simple: place a core sample tube
upside down on the surface – so the tube’s
opening was pressed into the soil – and drive the
tube into the surface using a hammer. For
Apollo 11, the issue was simply the design of the
core sample tubes. The tubes were designed
with a bevel to compact the soil and keep it
inside the tube. The lunar soil just below the
surface was already quite compact, and when
Aldrin repeatedly struck the 35 centimeter long
tube with the hammer, he reported that it would
go no further than eight or nine inches down.
Striking with increasing force, Aldrin actually
bent one of the extension handles before aborting
the task with a less-than-optimal sample. For
Apollo 12, the tubes were redesigned, and no
further issues were encountered for the
remainder of the program.
This highlights the challenge of core sample
collection on a very low-gravity body, however.
To illustrate, examine an astronaut attempting to
drive a core sample tube with a hammer on the
surface of Phobos. Recall that, for Phobos,
gTOT = 5.207 x 10-3 m/s2, and the assumed mass
of an astronaut in a spacesuit was 300 kg. This
means that the astronaut is held to the surface by
a force of just 1.56 Newtons. If a hammer with a
mass of 3 kg could was accelerated from rest to a
velocity of 10 m/s in one second, it would strike
a core sample tube with 30 Newtons of force.
While the tube will dissipate some of that
energy by sinking into the soil, the astronaut will
need to dissipate the remainder. It is not difficult
to envision that, by striking a core sample tube,
an astronaut will propel himself or herself into
the air, in a manner similar to that which was
analyzed in the Surface Mobility section.
Therefore, it is likely that if core samples are
desired, hardware whose operation can be
decoupled from the astronaut will be necessary.
Again, due to the multitude of NEO
compositions, and the specific nature of each
individual celestial body, the development of a
suite of tools to address this single problem will
be required.
This paper will present one
theoretical piece of hardware that, under specific
conditions, may suffice.
Figure 19 represents a tool designed to drive
a core sample tube into the surface of a NEO. Its
method of attachment to the surface is through a
chemical bond, so it would not be as effective if
used in a dusty environment, where it cannot
chemically adhere to the surface and provide a
reactionary force to the piston housed in the
upper chamber. The core sample tube would be
installed and held in place by a set of guide arms,
which would ensure proper alignment. The
astronaut would then press the tool to the
surface, where the pads would adhere. When
ready, the astronaut could them press a button
and the tool would begin driving the core sample
tube into the soil using the piston, which could
be powered by small, replaceable, high-pressure
gas cartridges. Upon the successful insertion of
the core sample tube, the astronaut would pull
the bottom tab from each pad, leaving a layer of
the chemical adhesive behind; the newly exposed
layer of pad would now be ready for use.
Piston
+ω
Pull Tabs
Core
Sample
Tube
NEO Surface
Figure 19. Theoretical example of a core sampling tool
This tool, however, would not necessarily
need to be used just with core sample tubes. If a
restraint system on the surface of a NEO was
desirable, this tool could be designed to also
drive pitons into the surface; the astronaut could
then route a tether through the piton, providing
some physical restraint.
As previously stated, Figure 19 is simply a
theoretical example of a tool that may aid an
astronaut in a number of different ways. As has
been learned through years of exceedingly
complex orbital EVAs, however, any tool or
hardware design meant to aid or simplify a task
for a spacewalking crewmember must remain
true to that purpose; tools and hardware that are
inherently complex often lack robustness,
resulting in crew time spent troubleshooting a
problem rather than accomplishing primary
objectives, but beyond that, complex hardware
often lack operability, leading to fatigue and
frustration over the tool’s use.
In an
environment as challenging as a NEO, tools and
hardware need to be designed, first and foremost,
for ease of operability.
14.4 Deep Core Sample Collection
Beginning with Apollo 15, the last three
Apollo missions brought with them a tool called
the Apollo Lunar Surface Drill (ALSD). The
drill was designed for two separate tasks. First,
it was designed to drill holes into the surface for
the Heat Flow Experiment. Additionally, it was
to take a deep core sample by driving the long,
segmented tube into the surface to a depth of 2.5
meters. The crew of Apollo 15 experienced the
difficulty in designing hardware for an
environment where prototype testing is not
feasible.
During EVA1, Scott reported encountering
some difficulty with the drill at a depth of just 30
centimeters while drilling the first of three holes
for the Heat Flow Experiment. Spending on the
order of 20 minutes troubleshooting this
problem, he was only able to get the drill to drive
to a reported depth of 170 centimeters. He then
proceeded to drill the second hole and
encountered the same problem. Due to his high
O2 usage, Mission Control called him off
drilling the third hole, leaving that task for
EVA2.
Upon returning to drilling tasks on EVA2,
Scott attempted to drill a core sample. A better
flute design allowed the sample tube to auger to
the full depth, but the hole became compacted
and Scott could not remove the core sample
stem. Again, high O2 usage forced him to
abandon the task to the next EVA. As a result of
the drilling problems, EVA2 fell about one hour,
forty minutes behind the nominal timeline.
During EVA3, Scott was assisted by Irwin
in attempting to remove the drill from the
compacted hole. After 10 minutes of very hard
work, they were able to extract it; in the process,
Scott sprained his shoulder. Scott reported that
drilling was the hardest EVA task he performed.
A new, improved drill design was flown on
Apollo 16, which allowed Charlie Duke to drill
his first Heat Flow Experiment hole the full
depth of 2.5 meters in just one minute.
Additionally, a jack-and-treadle system was
developed to aid in lifting the drill from each
hole.
Two items are immediately apparent when
considering the challenges encountered by
Apollo 15. First, any long-duration mission like
that to a NEO should carry with it enough
experiments to allow the crew to abort an
individual experiment if it is obvious that the
hardware design will significantly impact the
nominal EVA timeline. The number of days
available for proximity operations will vary
depending on the orbital dynamics of the target,
and as such, any time wasted troubleshooting
design flaws is just that, wasted.
Second, in a very low-gravity environment,
an astronaut will not be able to manipulate a drill
without a restraint system. If the spacecraft is
able to “land” and remain on the surface, an
astronaut may be able to operate a drill from a
foot restraint attached to the spacecraft, but
barring something this stable, it is not a task
worth considering.
There have been a number of papers written
that propose ways of mining asteroids [9,29]. If
the need for deep core samples is great, much
research will need to be done to determine the
best ways to operate drills on the surface of a
NEO independently from the astronauts. Even
the setup of something so massive will be a
daunting task for an unrestrained crewmember,
and thus it will be imperative to find ways to
conduct this sort of research with little to no aid
from the astronauts.
15. Geological Traverse
The first three missions to the surface of the
Moon relied on the astronauts on locomotion to
explore the region around the landing zone.
Apollo 11 remained within close proximity to
the LM for the duration of their single EVA. On
Apollo 12, the crew hiked approximately 4,300
feet during their second EVA to retrieve parts of
the robotic Surveyor III spacecraft, and on
Apollo 14’s second EVA, the crew covered
about three km on a roundtrip excursion to Cone
Crater.
Exploring by foot became a thing of the past
starting with Apollos 15. For that mission and
the subsequent missions, the astronauts were
able to explore the surface using the Lunar Rover
Vehicle (LRV). Apollo 15 used it for all three
EVAs, traveling as far as six km from the LM
during EVA3. Apollo 16 traveled as much as
11.4 km from the LM during their final EVA,
and Apollo 17 covered a roundtrip distance of
20.4 km during their second EVA.
For astronauts exploring a NEO, the
opportunity to traverse great distances using a
vehicle such as the LRV is not feasible, due to
the very low-gravity.
One could envision
intrepid engineers devising ways of using
propulsive units, or designing vehicles to operate
effectively in such a low-gravity environment,
but the complexity added by such simple tasks as
egressing and ingressing the vehicle every time
an astronaut wants to collect a sample may well
render this concept superfluous.
Additionally, although NEOs do have a
considerable size range, it is likely more
practical to spend an EVA in a specific area of
the NEO, and then have the spacecraft place the
astronauts on the following EVA in a different
area of interest. The advantage of this approach,
especially when one considers that the vast
majority of NEOs will be rotating, is that the
crew aboard the spacecraft can, through
observations and discussions with the scientific
teams on the ground, pick and choose the most
ideal location for each and every EVA.
However, the use of the LRV during the
Apollo program brings forth a question regarding
the maximum distance the astronauts should be
from the spacecraft; should an emergency arise,
the crew would need to be able to respond to the
emergency within a specific period of time to
minimize the risks to the spacewalking
crewmembers.
For the Constellation program, the ESAS
report was very clear on the limiting distance
[33].
If the tasks are located farther than the
emergency
return
walking
distance
(approximately 30 minutes), way stations to
provide suit consumable resupply should be
provided.
Thus, the astronauts should be no more than
30 minutes from the airlock, traveling at a
nominal speed on foot.
This 30-minute
constraint is a wise one, especially in a NEO
environment, where the surface composition may
well put the crewmembers at a heightened risk to
a spacesuit emergency.
Yet how does one quantify the translation
rates of the spacewalking astronauts when every
target will have a different gravitational force?
For the Moon, the experience of Apollo gave a
good indication for the speed with which an
astronaut could move without exerting excessive
energy. For NEO missions, it will be very
difficult to judge translation rates, especially
during the first few EVAs, and that lack of
knowledge will likely require that the astronauts
not venture as far as may be desirable.
16. Photographic Documentation
Photography has always been a part of the
space program. The images of boot prints
imprinted in the dust of the lunar surface, of
astronauts saluting the flag, of the entire sphere
of the Earth within the boundaries of a single
frame are iconic. Photography (for this paper,
photography really represents both still images
and video) has been the most effective publicrelations tool, connecting the vast majority back
on Earth to the few adventurers soaring the
heavens.
Photography is more than just inspirational
images, however. The astronauts of the Apollo
program used photographic documentation to
survey the various sites where samples were to
be collected, to provide the geological teams
back on the ground with context. By examining
not only the rocks themselves, when they were
brought back home, but also the context in which
they were discovered and collected, the scientific
teams could better interpret the history of the
Moon.
Aboard the ISS, the use of photography
plays a critical role in construction. After
completing a specific task – the change-out of an
orbital replacement unit, for example – the
astronauts are often directed to provide close-out
imagery of the worksite. This imagery provides
the ground teams with a complete understanding
of the configuration of thermal blankets and
access panels, allowing more accurate thermal
analyses.
And photography has become instrumental
in the space shuttle program since the Columbia
accident. Recall that Columbia broke apart
during entry in February, 2003, due to a hole in
the leading edge of the left wing, caused during
ascent two weeks earlier by a piece of foam from
the External Tank.
That damage went
undetected, and as Columbia passed through
entry-interface, plasma flowed into the cavity,
ultimately leading to catastrophic failure and the
loss of the vehicle and crew.
To ensure such damage did not go
undetected again, NASA developed a procedure
during space shuttle rendezvous called the
Rendezvous Pitch Maneuver (RPM). Positioned
directly under the ISS (and thus directly under
the windows in the US Laboratory Module and
the Russian Service Module, the space shuttle
would perform a 360 degree backflip, while
astronauts aboard the ISS would use digital still
cameras outfitted with 400mm and 800mm
lenses to take images of space shuttle. Those
photographs would be downlinked to an
inspection team on the ground, and that team
would analyze every image, searching for any
damage or misconfiguration. If an anomaly was
discovered, the appropriate action would be
taken to resolve the issue – STS-114 had to
remove a gap filler from the belly of the orbiter
during one EVA, and on STS-117, the crew had
to repair an OMS pod blanket using pins and a
surgical stapler [8].
These are but a few examples of the
effectiveness of photography to accomplish not
only public relations, but also critical science and
critical engineering tasks. For the astronauts of a
NEO mission, photography will also play a
critical role. The biggest difference between
prior missions and NEO missions will be the
astronauts’ capabilities to take high quality
images.
The lack of restraints on the surface of a
NEO will make photography a greater challenge.
Therefore, it will be essential to develop new
hardware, and new interfaces, to reduce the
workload placed on the astronauts for imagery.
One example that would be effective with the
Cricket System would be to integrate still image
and video capabilities into the spacesuit itself.
Cameras mounted on the spacesuit could be
controlled from an electronic pad worn on the
wrist, which would allow the astronaut to toggle
between cameras, zoom, pan and tilt. If the
cameras were integrated into a heads-up display
projected onto the visor of the helmet, an
astronaut could view his or her surroundings
through the various cameras, select the desired
angle, frame the image and take exceptional
pictures with little movement. This would
reduce the physical exertion required of an
astronaut, conserving energy, but it would also
allow an astronaut to stay in a stable position
while documenting the region in which he or she
is working.
This is just one possible solution to the issue
of properly outfitting the NEO astronauts with
the capabilities to perform high-quality
photographic documentation. What should not
be overlooked is the amount of energy expended
to conduct any operation during an EVA.
Pressurized gloves lead to hand and forearm
fatigue, and the impetus to complete all assigned
tasks often leads to something akin to tunnel
vision, where one loses track of time easily by
focusing on all the work that needs to be
completed.
The amount of energy required to retrieve a
camera, find a stable body position that allows
an image to be framed properly, take a requisite
number of photos, stow the camera and return to
the task at hand should not be overlooked. If the
required energy is too great, there is a high
likelihood that, as the EVA progresses and the
astronaut tire, these sorts of tasks will be
dismissed out of hand as not worth the effort.
This is not unique to photography, however.
This is a lesson that has been learned through
hundreds of spacewalks, and it proves the point
time and again that operational considerations
must be incorporated from the very beginning
into EVA hardware design; the success of
astronauts working on a NEO, the moons of
Mars or any other very low-gravity body will
require innovations in hardware and operational
concepts. By incorporating the EVA lessons
learned from the past 45 years, hardware
designers can create tools and spacesuits that
function in harmony with the spacewalking
astronauts; if they are not, much of the
astronauts’ preflight training, and much of their
energy during the spacewalks, will be spent
finding ways to work around the limitations of
the hardware.
17. Conclusions
The challenges over the next two decades
for human space exploration may revolve around
our ability, or lack thereof, to work in
environments that take the most difficult aspects
of lunar exploration and combine them with
those difficult aspects of microgravity EVAs in
low-Earth orbit. It will not be an easy task. This
paper has addressed just a small portion of the
challenges, by identifying some of the main
points of an exploration architecture, including
crew complement, surface mobility and field
geology. There is a seemingly endless amount
of open work.
The Apollo program, however, represents an
exceptional starting point, from which scientists
can develop experiments and mission designers
can develop operational concepts that allow the
goals of the science community to be met by the
men and women traversing these challenging
environments.
Additionally, as the author hopes he showed
in this paper, the experience gained through the
space shuttle and ISS programs cannot be
overlooked. These programs provide invaluable
information in identifying the most effective
ways in which astronauts work in the EVA
environment;
designing
hardware
and
operational concepts that take advantage of the
efficient modes of operation will provide the
greatest chance of success.
It is likely that many of these issues may
not be resolved by the time astronauts head off to
visit the first near-Earth object, but these issues,
along with a host of unknown unknowns that will
present themselves along the way, will require
resolution before we can begin to extend the
human presence farther out into the solar system.
Even in light of the broad challenges facing
it, a very low-gravity body exploration program
will provide great scientific benefit – many of
these low-gravity bodies are relics from the
formation of the solar system. Beyond that
obvious reason, however, two other reasons exist
for encouraging low-gravity exploration.
First, these potential targets may provide an
opportunity to develop in-situ resource
utilization techniques, allowing humans to more
easily explore farther out into the solar system.
If near-Earth objects, the moons of Mars, and
even main-belt asteroids could serve as refueling
depots, it would radically change the way
vehicles and mission profiles are designed.
Second, as mentioned earlier, a portion of
near-Earth objects cross Earth’s orbit. The ones
with a better probability of striking the Earth are
termed Potentially-Hazardous Objects (PHO)
[35]. Developing the ability to explore very lowgravity bodies leads to the ability to neutralize
the threat a PHO may present to the Earth. In
this way, a robust space exploration program
built around low-gravity body exploration
subsequently yields a robust planetary defense
program.
Mars is a lofty and worthwhile goal when it
is regarded within the context of solar system
exploration. To have ready access to potentially
hundreds of thousands of destinations, each
unique, each challenging, each valuable,
however, should be an ambition to great to pass
over.
Authors’ Note
This paper references a significant amount
of data from the Apollo program. As such, it is
impossible to annotate every such reference.
The authors would like to be sure it is clear that
all information in this paper from the Apollo
program came from two references.
One source was the Apollo Lunar Surface
Journal. It is edited by Mr. Eric M. Jones and
Mr. Ken Glover. The Journal is an incredible,
living document that is dedicated to accurately
capturing the vast amounts of data from the
Apollo program.
It is available at
www.hq.nasa.gov/alsj/frame.html. Note that this
source is in the reference list, but the only place
in the paper that it is referenced is the direct
quote from Mr. Armstrong during the EVA
technical debrief regarding surface operations
[15].
The second source, which provided concise
overview information for every Apollo EVA
comes from the Lunar and Planetary Institute.
This source is also listed in the references [23],
but again due to the vast amount of information
used from this website throughout the paper, no
other specific references are made.
The authors would like to thank the editors
and curators of these websites. Making the
technical information from the Apollo program
available to the public ensures that the lessons
learned from an historic program will not be lost
to posterity.
Acknowledgements
Matthew Gast would like to thank Georgia
Institute of Technology professor David Spencer
for the support, both moral and technical,
throughout the development of this paper. He
would also like to thank those members of
United Space Alliance and NASA who provided
a diligent technical and editorial review. Any
errors or oversight in content is the author’s
alone.
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